Install Pylint in client

This PR adds ``pylint`` to the client requirements, updates ``pylintrc`` to enforce more rules, and fixes some lint errors in both the client and server.

Unfortunately, the [git pylint commit hook](https://github.com/sebdah/git-pylint-commit-hook) is python2 only. So, under this PR, if you're in the client env then lint is not run automatically. Until we create a client hook ourselves, ``pylint`` will have to be run manually. 

If you're in the server env, then pylint is run on all changed files.