Add overload for lw_setup and tx/rx only setup

* Add an overload of lw_setup so that the caller can optionally specify
  which pins should be used for transmitting and receiving messages
  (and which interrupt to attach).
* Split setup into lw_tx_setup and lw_rx_setup so that these can be
  called individually for projects only using transmit or receive to
  reduce the compiled code size.
Merge pull request #3 from ribbons/specify-pins

Allow caller to choose transmit and receive pins