Refactor ability to use aliases for end points
Fix issue where allowing for aliases breaks v1 API
Fix uncaught exception when using an invalid cache key
Use DataFormatter to offer multiple response formats
Response formats for V2 API

- Implemented new formatters for JSON & XML output
- Refactored APIAliasTest to split out test DataObject and made fixture more
  generic for use in other tests
- Added format output tests and expected output files
Move formatters into own directory
Add test for invalid response format
Refactor formatter detection code in V2 controller
Expand tests
Fix typo in test error message
Add ability to paginate results
Refactor formatters and add unit tests for them
Allow API error messages to set the Content-Type header
Fix code style issue
Refactor how API errors are created
Update core release in travis config
Fix phpcs line length errors
Access expected format files using current absolute path
Refactor formatter for use with showResource action
Centralise API error messages and add additional unit tests