Add coverege

Author: SergeyTsaplin

Diff for: .travis.yml

@@ -6,5 +6,5 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
-install: "pip install pytest"
-script: py.test --doctest-modules
+install: "pip install pytest pytest-cov"
+script: py.test --doctest-modules --cov ./