Add support to embed videos.
Add 'on_demand' option to interact() so that long-running functions can be started only when explicitly requested
Disable run button until the function finishes
Add tests for the on_demand option
Add is_complete request and reply to messaging infrastructure
Expose IPython machinery for testing code completeness
Document is_complete messages

Also, bump message spec version
Merge master
Change from on_demand kwarg to interact_choose function
Remove add/remove class methods,
Remove helper set/get css methods
Fix tests
Initial crack at using specific traits for styling.
Partial implementation of styles
Finished style attributes.
Fab CSS
Updated widget examples for recent changes and,
renames from earlier PRs
Fix some bugs found by the widget examples,
add margin and padding base traits,
add overflow traits to box base.
Added Bootstrap specific classes,
and fixed a few bugs here and there...
Added bootstrap3 progress bar classes
Remove references to add/remove_class methods.