Allow user override of spacing and padding properties for alerts

This library is great. I recently found myself making an alert view where I wanted a full-bleed image in the content view (with a title above and buttons below). SDCAlertView was forcing an 18.5pt spacing between the bottom of the content view and the buttons, which I wanted to eliminate. I explored a few different options to change SDCAlertView, and this seemed to have the best balance of low-intrusiveness with flexibility. Let me know what you think, or if you've got a better thought for how to support this sort of alert.