Store encrypted email on Buyer (Bug 1028994)

- Add encrypted field to model
- Add field to resource
- Update API tests
Yeah that's what I tried at first but it's actually not long enough it seems, I kept getting truncation errors and the key field on BuyerPaypal in solitude uses longtext, so that's the only one that seemed to work.

| key      | longtext         | YES  |     | NULL    |                |