Add keyboard shortcuts to stage and upstage files

Stage is cmd-s, and upstage is cmd-u.

Conflicts:
	MGScopeBar
Set ⌥⌘A as keyboard shortcut for "Amend"
Merge branch 'master' into issues/156
Add pull request #156 to the change log
Merge branch 'issues/156'
Merge branch 'master' into issues/167
Add pull request #167 to the change log
Merge branch 'issues/167'