Injectable statsd client (#7138)

* Add ability to inject statsd client; some py test/reqs updates

- Updated the metrics logger to allow construction with an existing
statsd client, so that it can be configured by external systems or libs.
- added requirements to requirements-dev.txt which are needed to run
  tests-eg coverage, nose
- removed dependency on mock lib, it is in python stdlib now
- updated tox.ini to remove the now-superfluous deps

* add license to test file, and remove blank line at EOF

tests/email_tests.py

@@ -22,8 +22,7 @@ from email.mime.multipart import MIMEMultipart
 import logging
 import tempfile
 import unittest
-
-import mock
+from unittest import mock
 
 from superset import app
 from superset.utils import core as utils