add registrar binary, fix README

usage:

registrar -ip=my.local.address \
  -natsAddresses=1.2.3.4:4222
  -natsUsername=nats \
  -natsPassword=nats \
  -routes=8080:example.com,8081:example.org

Signed-off-by: Eric Malm <emalm@pivotallabs.com>
add registrar example to README

Signed-off-by: Eric Malm <emalm@pivotallabs.com>