Spot/883 implement exit method on ipython and python3 plugins.

this fixes a memory leak as old as the plugin: https://github.com/twosigma/beaker-notebook/issues/883

strangely, in order for the new ipython kernel API call to get through nginx i had to add a rule for it.
IMO there are too many nginx rules for ipython and that code should be cleaned up.

todo: update the other ipython based plugins (which they already need anyway)