two column quickhelp dialog, closes #3895

now that quickhelp is a modal dialog, it makes sense to have it be wider
and have the shortcuts take up two columns, instead of one. This change
makes that possible. See the results here:

![new quickhelp css](http://i.imgur.com/nvRTFJm.png)
Removed references to 0.11 and 0.12 from config/overview.rst
Uniformized prompt to
what's new tweaks

update nbconvert command-line call and add a few links.
Added explicit error message for missing configuration arguments.
explicitly load python mode before IPython mode

ensures IPython mode gets defined at startup,
which can sometimes fail because the Python mode was loaded dynamically.

closes #3636
Merge pull request #3911 from minrk/cm-ipython

explicitly load python mode before IPython mode
noteboook with spaces.ipynb test
updated test for spaces in filename
Merge pull request #3896 from ivanov/fix-3895

two column quickhelp dialog

fits on one page.

closes #3895
use notebook which has an image in it for the test
use grffile to support notebooks with spaces
add grffile to the basic latex template
move raw_input / %debug to higher prominence

in what's new
Merge pull request #3908 from minrk/whatsnew-nbconvert

what's new tweaks
change docs/examples refs to be just examples

this closes #3915
Merge pull request #3917 from ivanov/fix-3915

change docs/examples refs to be just examples

closes #3915
added check_for_tornado, closes #3916

This way, those relying on the output of `python setup.py install` get
clued in about the fact that it is required for the notebook
Merge pull request #3918 from ivanov/fix-3916

added check_for_tornado

closes #3916
Merge pull request #3913 from ivanov/alt-fix-3774

use grffile to support spaces in notebook names

closes #3774
closes #3898