allow num=0 in ratelimit

Currently the ratelimit function allows for bursts to occur up to the `num` limit.

I suggest to allow `num=0` in the ratelimit function. This would delay every data event (including the first one) that goes through the stream but unlike throttle would not drop any events.

Background:
I am using a recursive structure based on @vqvu 's code in https://github.com/caolan/highland/issues/371#issuecomment-196115715

There is currently no easy way to rate limit the main call in that example (i.e. the readdir call in the example - imagine that readdir being an API with a strict rate limit).

Start by locating the ratelimit function and reviewing how it handles the num limit and the first data event. Confirm the behavior against the recursive rate-limiting use case described in the issue. Done means num=0 delays every event without dropping any, unlike throttle.

Written by the indexing model from the issue text.