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`
autopep8?

Very, very minor, but this seems better to me:

```
    model_path = join(_my_dir, 'backward_compatibility', \
        'v0.9.17_test_summary_test_summary_LogisticRegression.{}.model' \
        .format(sys.version_info[0]))
```