Add about dialog in Notebook Help Menu.

This allow to get info on version of IPython when running remotely.
The about dialog also send a kernel info request and display the banner
which is useful for non-python kernel that don't match IPython version
Fix issue #5877 with tests

This pull request prevent to have lists environments inside the headings
cells which break latex code.

- First, the function `markdown2latex` has been changed in order to
  accept a markup keyword for pandoc reader, default to `markdown` as
  before(`test_markdown.py` updated too).
- Headings cells are treated with markup `markdown_strict+tex_math_dollars`
  to remove enhancements of pandoc for lists and add math support.
- Secondly, `prevent_list_blocks` filter escape characters `. - * +`
  in headings cells to prevent `enumerate` or `itemize` environments
  in latex sections(test added in `test_strings.py`

Test files accessible at:
    <https://gist.github.com/benjaminabel/4a40d1300dcf087c8695>
Build pandoc using haskell-platform in .travis.yml

This is required because travis use ubuntu 12.04 which ships with pandoc 1.9.11 this will not be required if travis update to ubuntu 14.04 whichuses version 1.12.2.1.
Get recent pandoc in travis from ppa cran2deb4ubuntu

This ppa:<https://launchpad.net/~marutter/+archive/ubuntu/c2d4u> contain
a backport of pandoc trusty package in precise. It can be used until
travis update his environment to ubuntu 14.04.
Move language info from kernelspec to kernel_info_reply
Use language_info instead of kernelspec in nbconvert
Remove language info from kernelspec test
Update docs about kernelspecs and kernel_info_reply
Add language_info field to message spec tests
cache sys-info
save future compile directives from ipython -i module.py
widget registry
registering core widgets
Adding custom keys for core jptr notebook
jupyter -> IPython
Merge pull request #6554 from Carreau/itsallaboutme

Add About IPython
fix Windows typo

introduced in #6810
Merge pull request #6819 from minrk/windows-typo

fix Windows typo
Merge pull request #6493 from SylvainCorlay/python_widget_registry

Creating a widget registry on the Python side.
Merge pull request #6793 from takluyver/langinfo

Move language info from kernelspec to kernel_info_reply