Basic user auth

Main changes:
Added devise and setup basic settings.
Added modals for sign in and sign up.
Some improvements in gemfile.
Poltergeist replaced with webkit.

Most of devise views - autogenerated, so need to discuss what we should do next. In most cases modals not enough: password change, password recover etc.
At this moment if you make error in a form rails will redirect to another page with sign_in/sign_up, but it possible to add ajax validation if we really need it.