Fix issue #258

I'm not 100% sure if this is the best way to go about it, but I've
tested and it seems to be working fine.

Some code snippets taken from :
http://core.trac.wordpress.org/ticket/14761
http://core.trac.wordpress.org/ticket/12629

Issue #258 states the custom post type "edd_payment", however I've
noticed pages are also created (as standard pages) which could do with
being removed (Purchase History, Checkout etc).

Will look into these later, would appreciate if could have some opinions
on this before I go any further ;)