#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.
Meh, I fear we can't have 2 comment threads on one line so I'll just comment down here.

Doing `kwargs.get` twice with some magic argument name building function is a bit weird. Replacing the loop with
```python
for entity_type in [...]:
    entity_ratings = kwargs.get(entity_type + '_ratings')
    if entity_ratings is not None:
       .... do what's done at the moment
```
would imho be nicer.

(My two comments apply to `make_tag_request` as well)