Add tox target to generate coverage reports

Author: lordmauve

requirements/cov.txt

@@ -0,0 +1 @@
+pytest-cov==2.5.1

tox.ini

@@ -29,3 +29,14 @@ commands =
     --profile-path={toxinidir} \
     --profile=prospector \
     --die-on-tool-error
+
+
+[testenv:coverage]
+deps =
+    -r{toxinidir}/requirements/pip.txt
+    -r{toxinidir}/requirements/cov.txt
+whitelist_externals = echo
+commands =
+    py.test --disable-pytest-warnings \
+        --cov-report=term --cov-report=html --cov=. {posargs}
+    echo Annotated HTML coverage report is in {toxinidir}/readthedocs/htmlcov/index.html