Do not rely on a leap webapp specific error.

This caused all tests to fail but even more would break compatibility with everything but the webapp.
refactor: Session::Document < CouchRest::Document

It also stores the database information now.

Database info was not stored properly before leading to this issue
on travis:
NoMethodError: undefined method `get' for "sessions":String

The database methods in Store were conflicting with the ones
included from CouchRest::Model. Now we are mimicing the way
CouchRest::Model works.
Merge pull request #4 from azul/bugfix/do-not-rely-on-leap-web-classes

Do not rely on a leap webapp specific error class.