Adding citation support.
Adding better logic to the PDF postprocessor.
Adding docs about latex citations.
Fixing review comments:

* Don't use lstrip/rstrip in that way.
* Renaming things in the pdf postprocessor.
Addressing review comments.

* Rename parse_citation to citation2latex.
* Add <p> block to test markdown.
Fixing docstring.
update example custom.js

fix event for at-page-load events
use font-awesome icon, not jquery-ui

closes #3998
Merge pull request #4143 from minrk/custom-icon

update example custom.js
Fixing attribute access.
Merge pull request #4090 from ellisonbg/citation

Add LaTeX citation handling to nbconvert