Added centerX & centerY property (equivalent to x & y)

Hi, I added `centerX` & `centerY` property which were familiar with old Three20's UIView category (sadly it's discontinued now).
I know there are `x` and `y` property already there, 
but they are a little confusing (not origin.x & y) and hard to search from Xcode's search navigator,
so I added as alternatives.

BTW I really like this category and I use in many of my cocoapods library.
Hope you'll merge this one. Cheers!