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.
Merge pull request #40 from redding/jcr-update-readme

Update the README to reflect the new DatTCP features