Modified skll_convert to be able to create ARFF files for regression.

* Added the `arff_regression` flag to `skll_convert`. Using this flag automatically removes any non-numeric attributes (like IDs) and also outputs the class label as `numeric` instead of as a set of nominal values
* Also added the `arff_relation` flag to `skll_convert` which allows the user to specify a relation name for the output ARFF file.