Add Humanized Errors to models
GithubAuthorization tweaks

Set request headers for API calls:

- Authorization: See http://developer.github.com/v3/#oauth2-token-sent-in-a-header
- User-Agent: See http://developer.github.com/v3/#user-agent-required

Remove `api_url_for_organizations` method
Set github omniauth scope to just user:email

See http://developer.github.com/v3/oauth/#scopes
Login page should be SessionsController#new :)