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`
hmm... sublimelinter complains there without the slashes.

actually, looking at pep8 guidelines again, i guess it should be the following, which is basically the same as autopep8's output except with the format call on one line.

```
    model_path = join(
        _my_dir, 'backward_compatibility',
        'v0.9.17_test_summary_test_summary_LogisticRegression.{}.model'
        .format(sys.version_info[0]))
```
Now we're really splitting hairs.