st2 & st3 compatibility

Most of the changes are brain-dead-simple with one hacky exception https://github.com/wuub/GitGutter/commit/f5d7fe45d357a66f399b4efaaa0ad0c5fa04fdcd

Tested on Win7 x64 & Ubuntu 12.10 x64. 
My OSX machine is still broken so someone else should verify if behaves correctly there.