Async style

This is a pretty radical change in interface, what I published as `nearley-stream`.

It's pretty node.js specific, though the actual number of lines to support the interface are small. It could be reworked to be more general, to work both ways. It mostly consisted of turning a while loop into something event-driven.

Right now, regular expressions in a grammar that match more than a character are not guaranteed to work: It can cross packet boundaries, and no attempt is made to work past that yet.

Thoughts are very welcome!