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.

Closes #36

@kellyredding - Ready for review.
@jcredding do you mean  "call the block for each" (not "call the block _method_ for each")?