URL for payment page should be more flexible

[groundwired]

To use a payment page definition, you have to specify the definition's salesforce id, like this:

https://gwop.na9.visual.force.com/apex/payment?pageid=XXXXXXX

I think it would be nice to also support calling the page by its name, such as:

https://gwop.na9.visual.force.com/apex/payment?page=Donation

It might also be nice, when no querystring is provided, to look for a page definition called Default and use that if it exists.