Add Blame support
repository: return a list from listall

Make listall_references() and listall_branches() return a list instead
of a tuple, as they're simply sequences of objects.
Merge remote-tracking branch 'petr/blame'
Fix tests with Python 2.6
Fix repository.write
Fix multivar interface.
Fix error handling in Repository.write
Merge remote-tracking branch 'xtao/multivar'
C coding style: indentation fixes
Update documentation
Get ready to release v0.20.0
Added methods to interact with the remote refspecs
Fix PEP-7
Use unicode for python v3 support
PEP-7: Do not use C++ style // one-line comments
Removed reduntant returns, fixs PEP7
Fix documentation
Refactor getters to become methods
Fix methods signature
Provide minimal diff option