Add request body + query params to matcher entry

For APIs where parameters are part of the URIs, httpretty works fine:

  HTTPretty.register_uri(HTTPretty.GET,
      'https://example.com/widgets/1/status', body='ok')
  HTTPretty.register_uri(HTTPretty.GET,
      'https://example.com/widgets/2/status', body='bad')

But when the parameters move to a POST body, the model of matching
requests by method and URI alone breaks down - we need to be able to
match on the body too:

  HTTPretty.register_uri(HTTPretty.POST,
      'https://example.com/widget-status',
      request_body='{"id": 1}', body='ok')
  HTTPretty.register_uri(HTTPretty.POST,
      'https://example.com/widget-status',
      request_body='{"id": 2}', body='bad')

Another problem crops up when we have multiple query string parameters,
and their values matter, but their order is arbitrary. This entry, for
example,

  HTTPretty.register_uri(HTTPretty.GET,
      'https://example.com/widgets?status=ok&sort=name',
      match_querystring=True, body='[5,81]'))

matches
      https://example.com/widgets?status=ok&sort=name
but not
      https://example.com/widgets?sort=name&status=ok

We can fix this by adding another match criterion by which we specify
the query string as a dict.

  HTTPretty.register_uri(HTTPretty.GET, 'https://example.com/widgets',
      query_params={'status': 'ok', 'sort': 'name'}, body='[5,81]'))

The logic of URIMatcher.get_next_entry is now generalized as: "Choose
the first matching entry that hasn't been used yet. If all matching
entries have been used, repeat the last one."