Make python code collapsible:

- Remove unused code: https://github.com/getpelican/pelican-plugins/issues/140
- Add javascript and CSS to header to toggle code areas
- load jinja2 template from file
add jinja2 template file to have collapsible python code
Merge branch 'master' into collapse-code
remove unnessecary replace from template, require white space after hash
get jquery via https from google's cdn
Maintain backward compatibility by not requiring that pelicanhtml is copied to the blog home
Explain new collapsible code cells feature in Readme.md
Update thumbnailer.py

A small quick and dirty (cf line 136) modifications in order to have the same directory structure than the picture directory.
Add possibility to keep picture dir structure

Add the possibility to keep the picture directory structure with the parameter THUMBNAIL_KEEP_DIR_STRUCT. The previous behavior is the same if the parameter is not precised
Add possibility to keep picture dir structure
Liquid Tags: line numbers for include_code
attempt to merge master with collapsible code
add the plugin to extract representative image for article and assign to an article's property
change image property name to featured_image
work with IPython 1.x and 2.0
Update Readme to specify which template is needed for which IPython version
Merge pull request #153 from joergdietrich/collapse-code

Collapsible notebook code cells in liquid_tags plugin
update Readme.md
Merge pull request #199 from krischer/master

Liquid Tags: line numbers for include_code
Merge pull request #200 from khuevu/master

Add a plugin to extract representative image for an article