Add pylint checks for Python code

Python got code formatting added in #326 - in addition we should add pylint checking as well to pick up common errors.

We could adopt the pylintrc from the [YAPF](https://github.com/google/yapf) project.

Note: The pylintrc and the YAPF style will need to agree - for example the indentation in YAPF is four spaces and the above pylintrc uses two spaces.

It will need to be added to the travis.yml, and all the issues (there will be lots) addressed.