ITS rule for pychan

Preserves string origin (xml files instead of guitranslations.py) to
help translators see the message context. We still need guitranslations
to actually load the translations however, so some strings will not be
translated at this point even though available as `translated` in the po
file.

ITS tool: http://itstool.org/

Use: `itstool -i development/pychan-its-rule.xml`