Support for deprecated UITextAttributes in iOS 7

This pull request does 2 things:
- Gives the user of the library a way to query what the keys are for accessing various text attribute parameters without manual inspection.
- Supports the NSFont text attribute keys that are recommended for iOS 7+ now that the UIText attributes are deprecated.

I started this PR mostly to start the conversation around support for this. Its possible that I have misinterpreted the reason for these methods and that another fix is desirable. Let me know what you think about this.

Thanks,

- Conrad