Migrate to pow

[eliknebel]

This PR changes the authentication mechanics over to use the Pow library and adds email support. A full list of changes can be found in #622. To summarize:

• Users can still register using supported social media provider or email
• Social media providers have changed from google and facebook, to now use google and github
• Users who register using email must verify their email address
• Users can now request to reset their password, in which case they will receive and email with a password reset link
• In development mode, "sent" emails can be viewed by going to https://localhost/dev/sent_emails. In production, email will be sent using Amazon SES
• Additionally, a better view of unit test coverage has been added using coveralls and integrated into github. Every pull request will now include a link to the unit test coverage results

Closes #622

[ThinkThoughtThunk]

Wow, this is great. There's a lot here. The one thing I saw is that editing a page shows the "default" user in the curriculum view when under active edit. It looks like the entry.ex live view is set up correctly to use the user's "given name" so I'm not sure what the issue is, and I don't know if this would happen outside a local environment, but wanted to bring it up.

[eliknebel]

That's an interesting one, I tried to recreate but was unsuccessful