Add 'git gui blame' command.

Much easier to work with than text blame when needing to navigate to parent
revisions and see full commit messages.

Automatically scrolls to the line on which the cursor is positioned.
Add menu item for 'git gui blame'.
Merge branch 'blame-gui' of https://github.com/rchl/sublime-text-2-git.git

Merged pull request:
#203: Add 'git gui blame' command.