Add anchor links to versions in the changelog (#191)

Uses [Sphinx syntax for cross-reference labels](https://stackoverflow.com/a/19543591/101923).

Author: tswast

docs/source/changelog.rst

@@ -1,6 +1,8 @@
 Changelog
 =========
 
+.. _changelog-0.5.0:
+
 0.5.0 / 2018-06-15
 ------------------
 
@@ -27,12 +29,16 @@ Internal changes
 - Refactored auth to an internal auth module. (:issue:`176`)
 - Add unit tests for ``get_credentials()``. (:issue:`184`)
 
+.. _changelog-0.4.1:
+
 0.4.1 / 2018-04-05
 ------------------
 
 - Only show ``verbose`` deprecation warning if Pandas version does not
   populate it. (:issue:`157`)
 
+.. _changelog-0.4.0:
+
 0.4.0 / 2018-04-03
 ------------------
 
@@ -50,6 +56,8 @@ Internal changes
     Messages use the logging module instead of printing progress directly to
     standard output. (:issue:`12`)
 
+.. _changelog-0.3.1:
+
 0.3.1 / 2018-02-13
 ------------------
 
@@ -58,20 +66,26 @@ Internal changes
 - Fix an issue where a dataframe containing both integer and floating point columns could not be uploaded with ``to_gbq`` (:issue:`116`)
 - ``to_gbq`` now uses ``to_csv`` to avoid manually looping over rows in a dataframe (should result in faster table uploads) (:issue:`96`)
 
+.. _changelog-0.3.0:
+
 0.3.0 / 2018-01-03
 ------------------
 
 - Use the `google-cloud-bigquery <https://googlecloudplatform.github.io/google-cloud-python/latest/bigquery/usage.html>`__ library for API calls. The ``google-cloud-bigquery`` package is a new dependency, and dependencies on ``google-api-python-client`` and ``httplib2`` are removed. See the `installation guide <https://pandas-gbq.readthedocs.io/en/latest/install.html#dependencies>`__ for more details.  (:issue:`93`)
 - Structs and arrays are now named properly (:issue:`23`) and BigQuery functions like ``array_agg`` no longer run into errors during type conversion (:issue:`22`).
 - :func:`to_gbq` now uses a load job instead of the streaming API. Remove ``StreamingInsertError`` class, as it is no longer used by :func:`to_gbq`. (:issue:`7`, :issue:`75`)
 
+.. _changelog-0.2.1:
+
 0.2.1 / 2017-11-27
 ------------------
 
 - :func:`read_gbq` now raises ``QueryTimeout`` if the request exceeds the ``query.timeoutMs`` value specified in the BigQuery configuration. (:issue:`76`)
 - Environment variable ``PANDAS_GBQ_CREDENTIALS_FILE`` can now be used to override the default location where the BigQuery user account credentials are stored. (:issue:`86`)
 - BigQuery user account credentials are now stored in an application-specific hidden user folder on the operating system. (:issue:`41`)
 
+.. _changelog-0.2.0:
+
 0.2.0 / 2017-07-24
 ------------------
 
@@ -81,21 +95,29 @@ Internal changes
 - :func:`read_gbq` now has a ``auth_local_webserver`` boolean argument for controlling whether to use web server or console flow when getting user credentials. Replaces `--noauth_local_webserver` command line argument. (:issue:`35`)
 - :func:`read_gbq` now displays the BigQuery Job ID and standard price in verbose output. (:issue:`70` and :issue:`71`)
 
+.. _changelog-0.1.6:
+
 0.1.6 / 2017-05-03
 ------------------
 
 - All gbq errors will simply be subclasses of ``ValueError`` and no longer inherit from the deprecated ``PandasError``.
 
+.. _changelog-0.1.4:
+
 0.1.4 / 2017-03-17
 ------------------
 
 - ``InvalidIndexColumn`` will be raised instead of ``InvalidColumnOrder`` in :func:`read_gbq` when the index column specified does not exist in the BigQuery schema. (:issue:`6`)
 
+.. _changelog-0.1.3:
+
 0.1.3 / 2017-03-04
 ------------------
 
 - Bug with appending to a BigQuery table where fields have modes (NULLABLE,REQUIRED,REPEATED) specified. These modes were compared versus the remote schema and writing a table via :func:`to_gbq` would previously raise. (:issue:`13`)
 
+.. _changelog-0.1.2:
+
 0.1.2 / 2017-02-23
 ------------------