Thread model and UI improvements

Adds thread model/collection for managing conversation-level state, such
as unreadCounts, group membership, thread order, etc... plus various UI
improvements enabled by thread model, including an improved compose
flow, and thread-destroy button.

Adds Whisper.notify for presenting messages to the user in an orderly
fashion. Currently using a growl-style fade in/out effect.

Also some housekeeping:
Cut up views into separate files.
Partial fix for formatTimestamp.
Tweaked buttons and other styles.