Fix static deletion methods and add tests

On https://github.com/recurly/recurly-client-php/issues/46 iclo pointed we have static delete methods calling into the non-static Recurly_Resource::_delete() causing a fatal error.

I moved the _delete() method up to Recurly_Base and made it static,
and adjusted all the deletion calls accordly. Transaction refunds are
a special case since they're the only place we do a DELETE that
returns a body. Since we had no tests, I spent a bit of time adding
those, which required some changes to pass the mock client through.