Introduced UserSessions (CONFIG CHANGE)

Only 9-12 months after they entered the architecture design, finally
began coding them up.

Moved GetThing into UserSession, which will soon begin to enhance the
SpaceState with UserValues.

Based on a suggestion from akka-user, began using the term "troupe" to
refer to "a bunch of closely related Actors".

IMPORTANT: added the session-dispatcher section to application.conf.

Minor tweak to the look of Conversations.