Add header menu spacing for markdown cells

fixes #449
Handle input menu when collapsed
wait until websockets in the ipython kernel are ready before sending the initialization code over them.  fix Issue #478.
Merge pull request #476 from twosigma/sjs/449

Add header menu spacing for markdown cells
Merge pull request #482 from twosigma/spot/478

wait until websockets in the ipython kernel are ready before sending the...
merge from master