Adds menuitem image capability with tests.

This only adds `NSMenuItem.image` capability and ignores `onStateImage`, `offStateImage`, and `mixedStateImage` since the discussion of those API methods in the Apple docs say: `Changing state images is currently unsupported in OS X. `. Maybe you guys know more about this functionality than I do.

Closes #6.

Please note that this will cause a merge conflict with #18, so whichever you merge first, I'll update the other PR and fix the merge conflicts.