Added xml parsing functions and ipython notebooks with tests of the
parser on the PR.xml PRL.xml files. Also included are html versions
of the notebooks.

New files
---------
apsparse.py: the parser functions
custom.css: css formatting for the ipython notebook html
parse-pr-xml.html/parse-pr-xml.ipynb: html and notebook version of
the PR.xml parsing.
parse-prl-xml.html/parse-prl-xml.ipynb: html and notebook version
of the PRL.xml parsing