Top Posts: add an option to choose what Post Types to display

This is only a first pass, trying to support CPTs as well.
I've also removed the code that excluded attachment pages,
since people would now be able to exclude them in the widget options.

Unfortunately, that patch doesn't work at the moment. I've been trying to understand why, but I must be missing something. I must not be saving the form properly. I would appreciate a second set of eyes on this.

Original trac ticket:
http://plugins.trac.wordpress.org/ticket/1797