[#66075840] Add Log-in/Log-out

This branch sets up postgres as our persistence layer and creates an 'account' model to represent a person/group with a replayio account who can log in and use the web app. It pulls in Devise to handle the actual work of logging in and out, as well as a few other gems I'll highlight:

App:
-PostGres: Our persistence layer. We shouldn't be doing much other than storing user accounts and subscriptions.
-Devise: Heavyweight gem that handles user creation/authentication

Testing:
-RSpec: Testing is awesome
-Capybara: Drives integration tests from the outside in
-FactoryGirl: Database setup for integration tests
-Guard: Runs your tests live as you save your files
-Database Cleaner: Scrubs the test database in between test examples.