add attributedMessage functionality

Hi - this is my first time doing a pull request on github, so please let me know if there is anything in this process that I can do better. 

The pull request code extends functionality for use of attributedText in the messages so that different text can have different font styles. (See the "Requested to Meet" and the "Accepted the meeting request" text in the image below) 

![attributedtext 2014-01-06 at 1 43 45 pm](https://f.cloud.github.com/assets/4063649/1854427/fee6d996-771b-11e3-925d-7780c18be418.png)

This is used through a method called "attributedTextForRowAtIndexPath" in lieu of "textForRowAtIndexPath".  If using attributedTextForRowAtIndexPath, the attributedTextFlag must be set to true, otherwise the code will default to using textForRowAtIndexPath