Make Volume Pricing Information Available Through the API

[janhartigan]

Per this: http://blog.balancedpayments.com/volume-pricing/

There are some informational pages on our site which indicate to our users that (in general) prices are lower for bank account purchases vs. credit cards. We'd like to show the users something like:

Bank Accounts: 1% + $0.30

Cards: 2.9% + $0.30

So the problem with this is that if our pricing changes, this information will be out of date. I believe that you can get this information through the invoices API call, but that's only relevant for the previous day (and according to some guys on IRC, this is something that may change from time to time). It would be nice to be able to just get my marketplace's current volume pricing through an immutable API call.

[steveklabnik]

Seems legit. Whenever we introduce #100, we'll want to expose the fee for the forex separately from our fee.