Refactor tag classes/structure

[thebigmunch]

This includes:

• [x] Create a Tag class.
  • Has attributes name and value.
  • Format-specific tag classes inherit from Tag.
  • All tag parsing logic moves to the appropriate tag classes.
• [ ] Refactor Tags classes to support both plain values and Tag instances in some way.

[thebigmunch]

So, after going down the rabbit hole of making everything in the Tags subclasses be a TagList, it really falls apart with ID3v2. It was some dark magic to begin with, but not so bad. But then having to support single-value frames, multiple-value frames, as well as multiple-occurrence frames, it got horrific.

But if instead we switch to not everything being a list, single-value and multiple-value can be handled inside the Tags subclass just fine. Meanwhile, multiple occurrence frames can be handled by a TagList different from the previously mentioned that presents an interface that can be handled the same way as a Tag in the ``Tags subclass.