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.
These new methods all use spaces instead of tabs. The project uses tabs and I'd like to keep that consistent. As far as I can tell, these four methods are the only ones where it's not right.