Update Interfacing.md

Two more updates: 
 - added instructions on installing pySerial, as that module is needed but commonly not installed;
 - removed the ser.open(); upon object creation the port is immediately opened, so trying to open it will result in an error that the port is already open. This should be tested on other systems, but I think this behaviour is system independent.
Merge pull request #26 from kimbauters/master

Update Interfacing.md