Merge pull request #1606 from mdboom/cxx_extension_policy

Document the C/C++ code guidelines
Initial work on WebAgg backend
mplot3d should call canvas.draw_idle()
Address documentation and smaller issues raised by @pelson in #1426.
Revert change to how width and height are handled -- the other was causing image resampling.
Handle slightly older versions of Firefox.
Handle non-binary image streams
Display URL if not set to open webbrowser
Remove some logging
Fix toolbar button events on Firefox.
Add message about stopping the server
Warn when using a browser that doesn't support binary websocket messages.
Add support for rubberband drawing.
Make the html backend prettier with ipython's css and switch port to 8988
remove empty ipython notebook
Add jquery ui hover hook for buttons
Move status message to a new line
PEP8 fixes.  Rename web_static -> web_backend
Indentation