Add SKLL to Python general purpose section

[SciKit-Learn Laboratory (SKLL)](https://github.com/EducationalTestingService/skll) is a Python package that makes it simpler to run batches of experiments with scikit-learn. It also interfaces with [GridMap](https://github.com/EducationalTestingService/gridmap) to allow you to automatically run a bunch of experiments in parallel on a cluster managed by DRMAA-compatible system like Grid Engine.