[#6461] Partial implementation of GC Issues importer based on gdata API

The gdata API was shut off, so to avoid unnecessary work, most of the
GDataAPI classes are left unimplemented and untested, but the core logic
in GoogleCodeTrackerImporter is implemented and tested.

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6461] remove gdata dep since it'll be replaced soon anyway
[#6461] fix pyflakes issues