Add CasperJS utility functions, and tests for code and
Markdown cell execution.

Code cell test is based on https://gist.github.com/ellisonbg/3329929.
Add a README for CasperJS tests.
Add a test to demonstrate errors when up/down arrow keys are
pressed in an empty notebook.
Use keycode constants from jQuery UI and improve message in test.
Wrap CasperJS tests in a helper function to reduce boilerplate.
Add tests for Markdown toolbar and menubar entries.
Use existing IPython method to kill kernels.
Add CasperJS tests for `merge_cell_above()` and `merge_cell_below()`.
jstest moved to new place, closes #3125
make casperjs test suite pass again
less convoluted way of grabbing output
consistently get the first cell
include the # of active kernels in server info
simplify prepare_controllers logic
added javascript section with casperjs requirement
JSController working, `iptest js` runs casperjs
start and use notebook server for js test suite
--no-browser flag to js tests notebook server
workaround for travis-ci/travis-cookbooks#155
don't try to delete notebooks

this method wasn't currently working, and though we should put a working
method there as a utility, it isn't strictly necessary, since the whole
directory will be deleted