Add getSignatures method for WeThePeople API

1) WeThePeople object is now able to call all 3 available web service
calls from the WeThePeople API -- updated spec to test each method
2) Standardized method signatures
-- each method first takes options (and maybe a petition id if
required), then a callback for data processing, then a boolean for
whether or not to mock the API call
3) Update routes to abide by API calls, and include signatures on a
given petition's page.