Refactor tests

[Pivotal Tracker](https://www.pivotaltracker.com/story/show/67243926)

* Replaced `selenium` with `poltergeist` for 99% of the tests
* Moved step for third party authentications to cucumber hooks
* Moved time stubbing to cucumber hooks
* Refactored steps
* Removed redundant steps
* Renamed `tabs.rb` to `tabs_steps.rb`
* Fixed spelling mistakes in steps
* Updated `README.md` notify users of the test dependency to `phantomjs`