fix design document protection in set_session and use utc

We caught the exception raised on a _design document and just proceeded.

Refactor: moved the build_or_update logic into CouchRest::Session::Document from the Store.
functions to cleanup the session store

We'll probably run cleanup_expired from a cronjob so outdated sessions do not clutter our database.
Also we have some ooold session records without an expiry set on some installs. So cleanup_never_expiring will help deal with them or reset all sessions on a system that does not expire it's sessions
refactor: separate removal of sessions from finding the expired ones
refactored and more verbose test
add design doc to couch on travis and document this setup
make tests more clear... hopefully
Merge pull request #2 from azul/feature/cleanup_expired_sessions

Feature/cleanup expired sessions