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`
@aoifecahill and I talked about having all the features and classes listed on the first line, but the concern there was that if you did `head foo.libsvm` you'd be in for a rude awakening if you had many features.  Maybe that's not a use case we need to worry about though.  Using JSON would certainly be simpler.