Improve coverage testing (#46)

* Add -f option when installing tarpaulin in CI

The `tarpaulin` README says "Because of the use of nightly proc-macro
features you'll need to reinstall tarpaulin each time unless you're
keeping to a specific nightly version. If you are keeping to a
specific nightly you can remove the `-f` flag in the example travis
file."

We haven't pinned a nightly version, so we should use the `-f` option.

* Add doc tests to coverage testing

Author: jturner314

Diff for: .travis.yml

@@ -16,7 +16,7 @@ matrix:
     - rust: nightly
 before_cache: |
   if [[ "$TRAVIS_RUST_VERSION" == nightly ]]; then
-    RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin
+    RUSTFLAGS="--cfg procmacro2_semver_exempt" cargo install cargo-tarpaulin -f
   fi
 before_script:
 - rustup component add rustfmt
@@ -30,6 +30,6 @@ script: |
   cargo test
 after_success: |
   if [[ "$TRAVIS_RUST_VERSION" == nightly ]]; then
-    cargo tarpaulin --out Xml
+    cargo tarpaulin --run-types Tests Doctests --out Xml
     bash <(curl -s https://codecov.io/bash)
   fi