Client for interacting with the Mixpanel Data Export API
query_data = %{"event" => "signed_up",
"from_date" => "2011-08-07",
"to_date" => "2011-08-09"}
auth_data = {"key", "secret_key"}
MixpanelDataClient.fetch("segmentation", query_data, auth_data)
=> %{"data" => %{...}}