Support for multiple categories/subcategories

[michalisFr]

This modification adds the ability to set multiple categories, each with an optional subcategory.

Category now accepts a single argument: categories, which is a list of tuples, each tuple being a category and subcategory couple. Subcategories are optional.
Example:
pod.categories = Category([("Arts",), ("Business", "Careers"), ("Government",), ("Fiction", "Comedy Fiction")])
The minimal entry when instantiating a Category object is a list of a single tuple with a single entry, eg:
pod.categories = Category([("Arts",)])

I've also updated the unit tests in test_category.py and test_podcast.py to succeed under the new schema.