Beginning to implemented threaded messaging

Inbox now shows only most recent message from each contact instead of
all messages. This isn't very exciting, but it's a start towards
threaded messaging.
Conversation activity & inbox item click added

Added blank template activity for individual conversation threads
Implemented onItemClick() for the inbox activity that launches the
conversation activity.
basic (non-functional) activity layout implemented
Conversation threads implemented

Touching an item in the inbox now takes you to the conversation view,
which is implemented. Messages are displayed in reverse chronological
order. The reply function is not yet implemented.