Refactor specs + fixes

- [x] Cleanup + Refactor spec syntax (these specs tend to be reused so why not keep them well styled).
- [x] Fixed Braintree a bit that had Test::Unit syntax in it, also add error notes to the pending tests.
- [x] Fixed SecurePayAU Gateway that had it active_merchant gateway misspelled.
- [x] Fixed UsaEpay spec.
- [x] Add base specs for 11 Gateways.
- [x] Updated README and Versionfile.
- [x] Bumped dependencies and made `braintree` a runtime dependency #114
- [x] Add badges for Travis and [Code Climate](https://codeclimate.com/github/spree/spree_gateway).

@radar ping! :squirrel:
p.s. I will continue write some more serious tests a bit later so please don't revoke those empty skrill tests. It still only on 63% coverage.