Support repos that use the .git-file mechanism.
Add tests for .git-file.
Merge branch 'feature/0.3/git-file-support' of https://github.com/igetgames/GitPython into igetgames-feature/0.3/git-file-support

Using the previous implementation of read_gitfile, but added the previously missing test from this PR.

Conflicts:
	git/repo/base.py
	git/test/test_repo.py
Merge branch 'igetgames-feature/0.3/git-file-support' into 0.3

Resolves https://github.com/gitpython-developers/GitPython/pull/89