Remove size config option in favor of manual classes

This is a proposal to remove the current "Icon Size" field configuration option, in favor of adding classes manually thru the text field.
There is a wide range of classes that could be added manually, but adding config options for each would be a lot to maintain: http://fontawesome.io/examples/

Instead of adding the single config option for size, you could just append classes to allow for more flexibility.
This can be accomplished by:
* Removing the size configuration option
* Use a Javascript function that preserves the extra classes when toggling thru the different primary fa-* options
* Set a default description with fontawesome_iconpicker_field_widget_form_alter() / $context['widget']->getPluginId() that provides explanation of appending with fontawesome's additional classes

I can assist in committing all these noted changes in a patch.