String height extension

This adds UIKit extensions on NSString and NSAttributedString to return the height of an (attributed) string with a given width (and font).

NSString:
\- heightWithWidth:font:

NSAttributedString:
\- heightWithWidth:

These are simply wrappers for existing NS(Attributed)String UIKit extensions:
\- boundingRectWithSize (NSAttributedString and NSString under iOS 7+)
\- sizeWithFont:containedToSize: (NSString under iOS <7).