GPG signature support on commit object.

I have rebased https://github.com/gitpython-developers/GitPython/pull/124 on current 0.3. Please review and consider to merge.
Originals:
Pull request "GPG signature support on commit object" #124 by Tatsuki Sugiura.
https://github.com/gitpython-developers/GitPython/pull/124
commit 8065d2abdbb18e09560fc061807301b4c834d5a7
commit 62ecd6c66a84144632b045696326af503ee8cd4e