Extend Task.rate_limit to accept float values

Celery is enforcing rate_limit integer values, but the kombu TokenBucket implementation
fill_rate value is casted to float anyway, therefore remove this limitation.

https://github.com/celery/kombu/blob/master/kombu/utils/limits.py#L40

Why? I really needed a more fine-grained rate_limit control (eg. 0.5/s especially for HTTP API requests, which are often throttled).

This change is backwards compatible, although I'm not sure if my patch can be merged "as is" into master, probably a few other places in the documentation need also to be modified.