Include a file from a git tag

This change allows inclusion of code from a specific git tagged version
of a file.  It looks for the given tag/filename in the local git repo;
it is not currently possible to specify a different repo.

The file is checked out into a temporary directory and included into the
book, and then the temp directory is deleted.  The operation thus has
no effect on the state of the repository.
Use FakeGit in tests
Extract CodeInclusion module into separate file
Merge pull request #103 from skmetz/include_file_by_git_tag

Include a file from a git tag