Text changes

Makes several changes to the pre-registration text based on feedback from AnthroCon.
Two minor nitpicky notes:

1) Instead of use the ``.total_cost`` property you could instead use the ``.dollar_amount`` property which won't require you to do this math.  Also, in general the ``.amount`` and ``.dollar_amount`` values are more correct, since the ``.total_cost`` attribute is just the default.  I should probably rename ``.total_cost`` to ``.default_total_cost`` since that would be more clear.  Also... maybe some documentation of this credit card code would be nice, since we process hundreds of thousands of dollars with it :/

2) If you are in a situation where you need to do integer division, be aware that in Python 3.0 and later, saying ``5 / 2`` gives you ``2.5`` and if you want to do integer division you should say ``5 // 2`` (which gives ``2``).  In previous versions of Python and also in most other languages dividing an integer by an integer always gives an integer, so this behavior is not always obvious.