skip a failing test on windows

soon, the cython magic will move into its own repo and this can be dealt
with properly, but most windows installs will not have a compiler
available out-of-the-box, so it's safer just to skip this altogether
fix incorrect `empty-docstring`

in notebook tooltip.

The else condition wasn't quite right, and would set doctsring to 'empty doctsring'
even when there was a doctoring.
Merge pull request #3849 from ivanov/windows-no-compiler

skip a failing test on windows

in cythonmagic
add missing store_history key to Notebook execute_requests
mention payloads for non-Python kernels

they should be ignored.
add log statement to PDFPostProcessor
clear template when template_file changes
moved get_ipython_cmd into separate function
use new get_ipython_cmd functionality
generate `export_foo` methods

and populate `exporters` dict mapping short names to classes
rollback singleton exporter instances in `export_foo`
create single exporter in NotebookApp

rather than using export_by_name
pass repr of a list on command line
Merge pull request #3836 from jhamrick/no-mathjax

Parse markdown correctly when mathjax is disabled

Closes #3835
Merge pull request #3848 from minrk/empty-docstring

fix incorrect `empty-docstring`
Merge pull request #3830 from minrk/pyout_metadata_doc

mention metadata / display_data similarity in pyout spec
don't run init_profile_dir twice
Merge pull request #3844 from minrk/payload

update payload source

should be simple key, not Python- (or worse, IPython-) specific long key.

I could have sworn I did this long ago, but apparently the source key was only updated in display_data, not execute_reply.
Merge pull request #3850 from minrk/store_history

add missing store_history key to Notebook execute_requests
moved TemporaryWorkingDir into utils.tempdir