Allow sphero gem to optionally use rs232 gem for serial communication

Hey there, your sphero library is great but I could not get it working on Windows using the serialport gem.  I was able to get it working with the rs232 gem after I made a patch (that patch has already been merged).  If you are interested I tried to make the sphero library configurable about which gem to use for serial port communication.  If you set the environment variable RUBY_SPHERO_RS232 it will use rs232, otherwise it will use serialport.  Let me know if you're interested in these changes and if there are any changes you'd like me to make first.  

Thanks again, 
Dan