Create a session service to store session data into cassandra

Summary:
Added a short string function

Update session factory Loads from a service_provider or defaults to php session

Added a BlackHoleSession (lasts 1 request)

Added session id trait and cass session obj

Test Plan: prefetch, delete, destroy, get and make sure it persists

Reviewers: brooke

Reviewed By: brooke

Maniphest Tasks: T44

Differential Revision: http://phabricator.cubex.io/D8
Removed short_string() function

We already had FileSystem::readRandomCharacters() so favoured this over
short_string() that was 5 character specific.
Available Keys
Added a url helper function

Shortcut to gettin to the urlSprintf method of the request object.