JS Configurablity Take 2

Change the way configurability works.
Config dict should be passed down to the parent class where it will be
merged with the default value and propagate to this only in the base
class.

This allow to both alter the configuration on a per instance basis, or
globaly by tempering with the class instance.

This also get rid of IPython global in some cases.
use hbox mixin instead of class

this will allow dynamic css in nbviewer like vbox-ing for low res
without JS or change in html
include hbox in outpuarea css
include vbox into .cell css definition
merge box-flex1 class into .output-area css definition
recompile css
backport If-Modified-Since fix from tornado

See facebook/tornado#713

closes #3177
Notebook actually gets and holds ename, not etype
VideoIframe from YouTubeVideo (simplified) + VimeoVideo
use much faster regexp for ansi coloring

ansispan function adapted from mmalecki/ansispan

The easiest way to see how slow the old way was is to create a very large traceback (e.g. max recursion), which bring everything to a screeching halt (even future page loads, if the output is saved).  Now a max recursion error draws in a second or two.

closes #3198
using named keys and format string method + renaming VideoIframe to VideoIFrame
add ignored *args to HasTraits constructor

makes subclassing nicer / easier

discussed [on the list](http://mail.scipy.org/pipermail/ipython-dev/2013-April/011409.html).
Update link to extensions index
Merge pull request #3208 from benjaminfjones/patch-1

Update link to extensions index
IFrame + ScribdDocument
tabulation adjustment
generic IFrame(src, with, height), refactored YouTubeVideo, VimeoVideo, ScribdDocument
Fix inheritance_diagram Sphinx extension for Sphinx 1.2
Don't crash if README_STARTUP is missing

On Windows inside a py2exe'd environment, README_STARTUP does not
get copied into library.zip. Added a warning and a skip if
the file IPython/config/profile/README_STARTUP is missing.
Merge pull request #3211 from takluyver/sphinx-xfileref-removed

Fix inheritance_diagram Sphinx extension for Sphinx 1.2