Allow players to register

[stevegrossi]

Thanks https://github.com/aaronrenner/phx_gen_auth!

At the start, this will just be an easy way to have your name automatically filled in when starting or joining a game, but eventually may provide a foundation for leaderboards, achievements, and more.

To Do

• [x] Send emails
• [x] Sign in with username or email

[yakryder]

Fixes #56