switch to the latest assert for test stubbing

The latest assert adds its own native stubbing API. The switches
to that. No behavior changes - just test cleanups.
Merge pull request #41 from redding/kr-assert

switch to the latest assert for test stubbing
Update the README to reflect the new DatTCP features

This updates the README to reflect the recent changes to DatTCP.
This includes changing the `Server` to a class from a module and
passing a block to `listen` to configure the TCP server. This
keeps the README relevant for reference when working with DatTCP.