Added new Tigris XML format bug importer/parser. Closes: http://openhatch.org/bugs/issue972

This patch adds a new bug importer for the Tigris XML format.

The "env/bin/py.test" runs successfully on my side. I ran pyflakes over the Tigris Python files without any problems showing up.
Additionally, I scraped the current state of the "SCons" Tigris database with the command:

  env/bin/scrapy runspider bugimporters/main.py  -a input_filename=/tmp/input-configuration.yaml  -s FEED_FORMAT=json -s FEED_URI=/tmp/results.json  -s LOG_FILE=/tmp/scrapy-log -s CONCURRENT_REQUESTS_PER_DOMAIN=1 -s CONCURRENT_REQUESTS=200

which seemed to work fine (2946 issues, in about 6:30min).
I hope the unit tests are sufficient for a start... ;)