better way to store json
Reformats and switches file-reading style
Pulls out `readFromRawResourceFile` into TestUtils class
Adds merchant_response.json
Drops checked IOException from TestUtils.read*
Adds missing / to resource name
Extracts merchant_users_response.json
Extracts all JSON from gc.api.Fixtures

There is a distressing amount of fragile exact-textual-comparison in the
tests that needs removing eventually (e.g. currently
`testPostPreAuthorizedBill` assumes JSON with no whitespace or line
breaks...!)
Fixes JSON indentation
Merge pull request #23 from gocardless/read_fixture_JSON_from_file

Reads JSON fixture data from external files