Vendored the activemerchant gem to help with testing

We needed a bogus paypal gateway as per
http://infotrope.net/2013/05/31/testing-paypal-express-with-activemerchants-bogusgateway-and-how-to-make-it-work/

Tests now pass usefully.
Removed docs directory.

It's not doing anything useful for us, and accidentally running "rdoc"
with the wrong command line args (as I just did) dumps a bunch of stuff
in it, which then pollute further commits. I'm going to gitignore it,
too (see next commit).
added doc/ to .gitignore