Upgrade testing tools

* Upgrades Pylint to version 1.1.0
* Adds diff-cover to travis builds (also runnable in a local script, `./scripts/coverage`)
* Adds diff-quality to travis builds (also runnable in a local script, `./scripts/quality`)
* Adds a maximum pep8 violations script, mirroring the maximum pylint violations script
* Fixes numerous Pylint errors