PaymentMethod utility class

The immediate gain is that utm_source payment method will be
constrained to sane values.  The payment method component will be
the name of the payment submethod "group", with an "r" prepended
for recurring donations.

There are also two long-term goals which are supported by this patch:
* Selecting the gateway by desired payment method
* Unique and meaningful designation for each payment method

Change-Id: I34602faa9d8cca99132aa3b8067247e0f0a58b9e
Merge "PaymentMethod utility class"