Remove OmniAuth dependency

Fixes #13 I think.

The OmniAuth stuff is great, but if people just want email/password auth this will allow them to have that without having OmniAuth in their Gemfile. The other solution would be to add OmniAuth as a gem dependency, but it seems silly to add another gem if you don't need to.

Anyway, I ran tests prior to submit -- and some failed -- but then I looked at the build badge and it appears to be the same ones that are failing on Travis CI, and don't appear related to my changes.

Thanks for making this awesome gem! We just started implementing auth in an Angular/Rails project and thought it would take for ever-- and instead we'll have it done in a night thanks to your awesome work :)