Simplify query string generation with the "qs" module

qs is the same module used to parse URLs within Superagent, so using it
here provides a more robust solution to the issue of indexed array syntax:
instead of planning for the request library to transform our URI in a
particular way, format our URI in that manner to begin with so that no
further transformation occurs.

The `prepareFilters` and `generateQueryString` methods were removed: their
behavior is replicated more simply using qs.