Add full support for .libsvm files

The main change for this PR is that `.libsvm` is now a valid format for `skll_convert` and `run_experiment`.  

There are also some other fixes I added along the way:
- Add PyYAML to requirements.txt
- Fix license headers in the `cli` package. They were all still GPL for some reason.
- PEP8 issues throughout `test_skll.py`
A slightly more verbose comment here would be helpful.  Why is this necessary?