Initial version of Donation schema

This is an initial schema for donations. It's based on some work we've done here at Trilogy.

I consider the suggested schema to be a conversation starting point. While I like most of what's here, there are several aspects that I haven't addressed that should be discussed and others that need fleshing out. As a starting point we definitely need to discuss:

- Transaction level data and additional payment data such as payment gateway responses, ids, etc
- Refunds/credits/voids (there's basic support there now)
- Bundling or line items (probably beyond the scope, but we should discuss)
- Purchases such as tickets, etc (probably beyond the scope as well)
- Recurrence (basic support is there)
- Use of identifiers
- Should source and attribution be arrays?