Add support for long-pressing links

I needed to display UIActionSheets when a user long-presses a link, so I added long-press detection and delegate methods to TTTAttributedLabel. I've been using this code for a couple months and it is stable.

A few notes:
 - I exposed the `linkAtPoint:` method in the header file.  In my app, long-pressing on a table cell has one action, and long-pressing on a link has another.  This allows a developer to pass touch events on non-link portions of TTTAttributedLabel up the responder chain.
 - The `didLongPress` versions of the delegate methods also provide the point at which the user tapped the link.  This is helpful when developing for iPad, where a UIActionSheet needs to be presented from a specific rect.

I initially thought these changes might be too app-specific to submit, but other label projects have similar functionality, so I thought I'd offer it up.