Clear_output: Animation & widget related changes.

- No more delay when calling clear_output
- Removed ability to clear stderr and stdout individually.
- Output div height is unmodified
- New wait flag prevents clear_output from being executed until new output is available

Animation flickering also mentioned in #1563

Some of the behaviors that were unwanted can be seen in [this YouTube video](http://youtu.be/dLWzAt-DDao).