Allowing carbon_writer to send via tcp or udp.

* Added new configuration 'LineReceiverProtocol' option for carbon_writer.
  Accepting string values of "tcp or "udp". carbon_writer.py defaults to
  "tcp" if this confirmation parameter is not present, this should allow
  a seamless - no config change request - upgrade for users if they sync
  to a new version of collectd-carbon.
* changes to carbon_writer.py : conditional checks to send metrics via
  SOCK_STREAM or SOCK_DGRAM depending on LineReceiverProtocol value.
* changes to README.md : describing new config parameter