Devise authentication

Added Devise authentication to require a login for creating questions and liking / unliking questions

I only realised that @yokree had already done a lot of this in askaway/askaway#20 after making this PR - so I've pull his branch and merged it into this.

@yokree's modals for logging in and registering actually look pretty swish.

Also, although there is now authentication on the liking and unliking controller actions, those are triggered by Ajax, so a redirection is not as useful as a status code and some JavaScript error message.

Because of the discussion on askaway/askaway#18 I haven't fixed that.