Wrap file created by mkstemp

[dougluce]

This promotes the FakeFileWrapper class to top-level and has mkstemp wrap its file. This lets fdopen() work properly. This should address #23 (among other things).

[jmcgeheeiv]

Thank you for this fix. Could I ask you to address these?

• Can class FakeFileWrapper be returned to its original place in the file so that the diff is smaller?
• Please add a test. fake_tempfile is using AddOpenFile incorrectly. #23 provides a fine start.

[dougluce]

I don't think FakeFileWrapper can be returned to its original place as it was promoted and unnested from FakeFileOpen. If it helps, this is the diff of the other changes to that class:

12c12
<   def __init__(self, file_object, update=False, read=False, append=False,
---
>   def __init__(self, file_object, file_path, update=False, read=False, append=False,
15a16
>     self._file_path = file_path
196c197
<       raise FakeLargeFileIoException(file_path)
---
>       raise FakeLargeFileIoException(self._file_path)

I extended the testMkstemp test in fake_tempfile_test.py to cover this case, adapted from the description on #23. This test fails on 37fb868 (as is proper).