Fix README: do not use interpreted text roles.

These are not understood by the PyPI rst converter.
Enhance error message of NoSuchRemoteError exception.
Enhance error message of BranchExistsError exception.
Fix: GitFlow.publish() does not return the branch name.

This fixes issue #6.
Fixes to doc-strings.
Minor correction in DEVELOPERS.rst.
Add a test-case for GitFlow.publish() returning the branch name.

See 2013-04-29T18:05:58Z!h.goebel@crazy-compilers.com.