Add EvidenceDetails object
General evidence test cleanup.

- Assert equality/nullity directly.
- Run tests about evidence disputes on each build[0].
- Fix typos in object types within the method.
- The card used in the dispute expires on 12/2014. Push it to 2020

[0] It'll be much slower to run the tests, but it's much better to
run tests than not.
Extract utility method in test.
Added test that setting and saving dispute evidence works.
Feature parity between stripe's docs/api wrt dispute evidence
Make REFUND_LIST_TYPE into a constant