Added support for truncation

DTCoreTextLayoutFrame - added truncation support w/ attributed truncation string

DTAttributedTextContentView - added flag to disable automatic height adjustment. This allow us to subclass, disable resizing, enable truncation, and have a DTAttributedTextContentView that behaves like a UILabel.

Change-Id: Ibb3231c3c58f2650f9358a5e8297ab45b7d792d6
Merge pull request #274 from bkennyatmeetme/master

Truncation support so DTAttributedTextContentView subclasses can  used like UILabels