Add ability to force a push

when we try to reset a branch that is available remotely, git throws an exception as it should be and SourceTree captures that message and shows it, but does not gives up any chance to force push the branch...

here's a comparison with Git Extensions:

http://s6.postimg.org/yakd1kzdt/screenshot_80.png

even when I'm trying to push the branch back to origin the "force push" checkbox is disabled

http://s6.postimg.org/i0u6yoopt/screenshot_81.png

using Git Extensions, it gives us the option to actually pull, rebase, merge, force the push or simply cancel the command...