Adding custom errors for 404 & 500

[Pivotal Tracker story](https://www.pivotaltracker.com/story/show/64956494)
- Added `errors_controller` with methods for `not_found` & `internal_error`
- Added `customerror.feature`
- Modified som step defs (needs refactoring see [chore](https://www.pivotaltracker.com/story/show/65548360)
- Changed settings in `config/enviroment/test.rb` & `config/enviroment/production.rb`
`config.consider_all_requests_local       = false`
`config.action_controller.perform_caching = false`

### And we had good time doing it! ;-)