Support iOS 6

From what I can see, there is no reason not to support iOS 6
Make setBarTintGradientColors: more defensive. Remove an unneeded to constant.
Fix the insertion of the gradient layer causing breakage in title and buttons.
In iOS 7 the gradient layer needs to be put back at layer 1, it seems.
Fix certain changes I made to pod spec that I did not need to do.
Merge pull request #2 from NebulaFox/master

Support iOS 6