Add test-case for triggering issue fixed in pull-request 5.

I'll enhance the test-case just after I've fixed the issue.
Update gitflow/core.py

I only know GitPython very vaguely but this looks more correct to me.
Enhance test-cases for finish w/ fetch.

This also includes a work-around to a bug in GitPython 0.3.2 RC1
which is unable to parse FETCH_HEAD reference files.
Finished feature pull-5.
Fix a typo.