Make it easier to develop locally (#1748)

Previously the only documentation for how to run tests was the
implementation in the Travis configuration file. Here we add
instructions as well as move development dependencies to an easily used
and shared (with Travis config) separate requirements.dev.txt file.

requirements.dev.txt

@@ -0,0 +1,8 @@
+psycopg2-binary
+behave
+coverage
+flake8
+mock
+pytest-cov
+pytest
+setuptools