An initial implementation of a message-based interact update system.

One problem still seems to be that the stdout reply messages are replying to the original execute_request, rather than the message handler.
More work on interacts using ipython message handlers
Merge branch 'master' into interact-messages
Better debugging for forking kernel errors
Add metadata key for display_message messages
Add a convenience function that will find the Sage cell session, given a div in the output region
add a debug flag to the web server
Add some experimental code for namespace interacts
Refactor display_data handlers into a dispatch table
More experiments with interactive controls

The slider, input, and pythoncode controls seem to work decently well now.
Revamp namespace interact python code
Clean up namespace interacts
Consolidate sage.util.createElement calls
Set up dispatch table for new interact controls
Fix == to ===
Get rid of extra unneeded argument
More cleanup of namespace interact controls
Remove the control_id for the update javascript function
Fix renaming problem
More fixes for expression boxes