Allow developers to use a custom font as well as FontAwesome font.

Dave Grady, I don't know if you are open to pull requests on this, but I found your library today and would like to use it intact as a cocoapod.  I need the flexibility to specify my own custom font when an image that I need isn't in FontAwesome.  I didn't see how I could enable this by subclassing.  Maybe a few other developers would like the option as well.
-Doug Ivers