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.
Fixing attribute access.
Merge pull request #4090 from ellisonbg/citation

Add LaTeX citation handling to nbconvert