#145: submit tags and ratings for all entities that support them

Currently just uses lists of supporting entities, no unified data structure. Also improves the docstrings a bit, including how to remove ratings and tags.
This removes a tiny bit of safety because it's now possible to use bogus kwargs (like `artistrating`, `artistratings` and I would be a liar if I told you I've never dropped an underscore somewhere :) ).

Since we're not iterating over `kwargs`, something like the following should suffice:
```python
for entity_type in [...]:
    if ....:
        ... do what's done at the moment ...
    kwargs.pop(entity_type)
if kwargs.keys():
   raise TypeError(....) # This is what Python itself raises for invalid kwargs
```