Add Mongoid support

Add Mongoid as development dependency
Add, enable MongoidAdapter

Thanks to @mediatainment who contributed the "Mongoid support" part
of this this commit a long time before it became production-ready and
who litterally ensured the support for users in the meantime.
See https://github.com/mediatainment/simple_token_authentication/ \
commit/6b30379ebed0f277487b63f3ec1e29129d598196

Fixes #38