Changed set_nav_bar_button to accept a system_icon hash key that allows creation of system buttons like add.
Merge branch 'master' of https://github.com/sxross/ProMotion

Conflicts:
	lib/ProMotion/screens/_screen_module.rb
Fixed system item in set_bar_bar_button to take UIImage change into account; updated README
Merge pull request #73 from sxross/master

set_nav_bar_button_item addition (again)

Thanks @sxross!