Don't use Rake for the benchmarking scripts

This switches from using Rake to run the benchmark scripts to
just using ruby script files. This minimizes using Rake with the
gem and only requires it for gem building. I'd rather not rely on
extra dependencies that are not needed and for this case, Rake
isn't needed.

This removes the "run X requests against the Benchmark server"
functionality. This hasn't been used since DatTCP has been stable
and isn't related to "benchmarking". It was more of a system test
to see that everything worked for an echo server.
Merge pull request #39 from redding/jcr-dont-use-rake-for-bench

Don't use Rake for the benchmarking scripts
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.