Adds menuitem image capability with tests.
Fix merge errors from another merge into master.

* master:
  Support custom views for NSMenuItems

Conflicts:
	lib/menu-motion/menu_item.rb
	spec/menu_motion/menu_item_spec.rb
Merge branch 'MohawkApps-image'
I don't like single letter variable names, even if 'value' isn't much better