Use serial_for_url to connect

I'm using TIC controlled pumps for receivers on the Submillimeter Array via an ethernet to serial bridge, so I needed to be able to connect via a socket connection. By using the pyserial serial_for_url factory function, any of the pyserial supported connection types can be used (the baudrate and other settings get ignored for connections that don't support those). I've tested this change on the hardware in our lab, but not with a local physical serial port.

Switch from using serial.Serial() to serial.serial_for_url() to open connection, so that the socket:// protocol and other pyserial supported protocols can also be used.

Closes PaulKGrimes/edwardsserial#1