Add functionality for specifying an NSAttributedString to use for the truncation token

Add functionality for specifying an NSAttributedString to use for the truncation token with a new property: ` truncationTokenAttributedString`. This allows for a bit more customization of the truncation token, such as inclusion of different fonts, colours or anything else you would do with an NSAttributedString.

For example:

![screen shot 2013-12-19 at 4 53 24 pm](https://f.cloud.github.com/assets/708741/1786645/d569ba4c-68f8-11e3-9567-94939ddcb4c8.png)