Add support for glyphicons/fontawesome.

A new `icon` parameter can be used instead `image` to use a glyphicon or font-awesome icon. Also, additional classes can be added to it through the `class_icon` parameter, like `text-success` ... `text-warning` bootstrap classes.

Default icon set is glyphicon, and can be changed to font-awesome using:
``` $.gritter.options.icon_prefix = 'fa'; ```