enable configuration options to enable/disable different authentication providers

[seafoodbuffet]

No description provided.

[sahat]

@seafoodbuffet I will merge this PR tomorrow. There are few things I wanted to modify first. Instead of facebookAuth: true, githubAuth: true, etc. scattered throughout secrets.js, have an object auth = {} that contains facebook, github, google, twitter, local booleans. Or perhaps an array of strings. But object would be easier to check for presence of authentication methods.

[sahat]

@seafoodbuffet I will fix it this time, but in the future could you use spaces instead of tabs in Jade templates? If you mix both, it will throw a 500 error: Invalid indentation, you can use tabs or spaces but not both.