Text changes

Makes several changes to the pre-registration text based on feedback from AnthroCon.
You could replace this with

``raise HTTPRedirect(''paid_preregistrations?payment_received={}', charge.total_cost)``

and the ``HTTPRedirect`` constructor will interpolate the variable for you.  There's not much difference in this case, but if you're interpolating a string, the constructor will also urlescape the string.