Replaces pry-rails with pry
Adds gem file back into Gemfile
Adds validations to user.rb & tests in user_spec
Migrations:
* Adds password digest
* Indexes users
Defines ruby version in Gemfile & adds unicorn
	for improved server performance on heroku
Changes style for error messages (WIP)
Changes to users_controller & users/form
	(adds password to params)
Changes to sign_up_spec (WIP)
Comments out links temporarily (WIP)