Payments page: client now sends cents value to server

I have updated the payments page to convert the dollar amount input by the user into cents before it is sent to the server. Conversely, it also takes the cents amount returned by the server (in case of a validation error etc) and convert it back to dollars.

The alternative solution would be to parse the dollar figure on the server, however I think this method is neater.

This fixes issue #24 #25.