Major changes to adapt refinerycms-members for Refinery 2+

Hello!
This pull request is a major update for the library. Basically, everything was rewritten to work with the latest gem versions of Refinery (2.0.0+) and its dependencies.

You should probably bump a major version of the gem. README file already mentions that this gem is not compatible with RefineryCMS below 2.0.0.

There's a few tweaks I'm still working on (e.g. I'd like to add "needs_login" checkbox to the Page form without overriding it -- it's missing now and requires that end users add it manually), but the library is ready to go as-is.