add posix_path filter

mainly for unixy things on Windows (latex) that don't allow Windows paths.
ensure filenames have posix-paths in latex on Windows
add missing import, properly use os.path.join
fix #3889
Merge pull request #4 from ivanov/posix_path

add missing import, properly use os.path.join
Merge pull request #3888 from minrk/posix_path

posixify paths for Windows latex

closes #3823, #3885
Fix documentation of Kernel.stop_channels
Merge pull request #3892 from andreabedini/patch-1

Fix documentation of Kernel.stop_channels
Merge pull request #3891 from ivanov/fix-3889

fix #3889
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.
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