Fix Google Code Bug Importer (issue871)

This is quite a large change, but it fully addresses the deprecated issue tracker API for google hosted projects. I've reworked a the importer to use successive calls to get CSV data, as this will give us most of the data we need. The only additional calls that need to be made via scrapy are for the issue detail pages themselves to get the full issue summary.

There will also be a pull request for the oh-mainline repo to fix the tracker model URLs in the YAML files.