First run at analyzing collections of petitions. I've had to limit the number of petitions to < 1000 in order for Semantria to respond. It's hard-coded to 100 for now, but should soon be replaced by arbitrary parameters on petitions (e.g. issues, creation dates, statuses).

Also:
* DRY up petition id junk in EnhancerWorker and assert that the analysis_complete flag is set.
Add issue and status criteria to the "petitions" resource request parameters.
Add support for signatures as criteria for petitions.