Minor release of python-weka-wrapper3

Today, I incorporated some enhancements that I had collected over the last few months for my python-weka-wrapper3 library:

• It is now possible to specify the class attribute via its index when loading a dataset (Loader.load_file or load_any_file), rather than having to set the index afterwards
• MultiFilter and MultipleClassifiersCombiner now have a clear and append method for faster manipulation of filters/classifiers.
• Creating subsets from datasets is now easier with the subset method of the Instances class: it allows you to generate subsets of columns (by name or range) and/or rows (by range)
• You can also retrieve all the attribute names now from an Instances object via the attribute_names() method.

Nothing earth shattering, but some useful improvements to make code more concise while maintaining readability.

The latest release is: 0.1.13