Use one code for two users.

This is not ready for merging yet, but I wanted to get something out there.

* Adds few dependencies (faker, rspec, factory_girl, sqlite3)
* Adds rspec test framework
* Changes the test and development environments to use sqlite3
* Removes paired_code_id field
* Adds tests for code (soon to be user)
* Add User#has_pair? & User#pair