#752 Autolayout support for DTAttributedTextContentView

Solve the issue #752

I added invalidateIntrinsicContentSize to setAttributedString: to give notion to Autolayout that an updates of the layout is required.

In my codebase using [view systemLayoutSizeFittingSize:UILayoutFittingCompressedSize]; i was able to get the expected size in iOS7 but not in iOS6. Now with invalidateIntrinsicContentSize it works in both.