Add support for conditional formatting rules

Based on work done by Amin Mirzaee, see
https://bitbucket.org/aminland/openpyxl/commits/b3fcdfc75ac2c6c9c5510bb3d9c3f8ec55c42cec
for original commit.
Add .gitignore file
Fix for newer versions of xml.sax.saxutils that default to charref replace