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)
don't force . relative path, fix #3897

As per #3897, .\filename is not recognized by pdflatex.
Merge pull request #3901 from ivanov/fix-3897

don't force . relative path, fix #3897
what's new tweaks

update nbconvert command-line call and add a few links.
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
Merge pull request #3896 from ivanov/fix-3895

two column quickhelp dialog

fits on one page.

closes #3895
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