close websocket connections on ping/pong timeout

we recently added ws pings for keep alive,
and we can now use the longs to close dead connections.

This can apparently happen when connections are not closed cleanly,
e.g. on VPNs.

Setting ws_ping_timeout = 0 disables the ping mechanism altogether.
Add Math display support for qtconsole
Allow output prompt to leave out numbers for qtconsole
Initial messing around.

Latex tab completion will have to be done outside the normal
completer logic as the completer line splitting logic uses
\\ as a special character to split lines on. I probably want
to put the latex completions first and it if finds any matches,
don't do any other completion logic. The only issue is that might
short circuit dir/path matching on windows. Hmmm.
fix directory listing with broken symlinks

directory listing would 404 if any listed file doesn't exist
(e.g. symlink to missing target).

Previously-failing test included.
add secondary `debug` flag for heartbeats

in both KernelRestarter and parallel HeartMonitor

Since these both produce a huge amount of output,
they limit the usefulness of `--debug` output for debugging
other aspects of IPython.
Strip the IPY_MODEL_ prefix from widget IDs before referencing them.
test for overly greedy completion swallowing
fix for underscore completion
Adding autogenerated latex_symbols.py.
Adding regexp for unicode identifers to CM mode.
Initial implementation of latex->unicode completer.
Cleaning up and only doing latex completion for python 3.
Adding partial latex completions.
Adding tests and limiting CM mode to python 3.
Adding latex_symbols.ipynb.
Adding script to generate latex_symbols.py.
Updating gen_latex_symbols.py
update mathjax CDN url in nbconvert template
update regex