fixes issue #335
A patch to extend a cookie based session, when a user accesses sync gateway after 10% of the session expiration has elapsed, the original session ttl is added to the current system time and the session and cookie are updated and new cookie is returned to the client.

A new test has been added to the admin api
Improved readability of cookie expiration tetsts
minor formatting fix
Update session.ttl to store string formatted duration
Changed variable sessionPercentageElapsed to sessionTimeElapsed
Now uses time.Duration type for session Ttl, handles the case where there is no existing Ttl entry in a session, in this case Ttl is set to 24Hours
Moved default session ttl to const
Merge pull request #365 from couchbase/feature/issue_335

fixes #252: Auto-renew session cookies