Implement a ReadWriteCloser; add a WriteByte() method

Hi Dave,

I modified your i2c library to implement a ReadWriteCloser so that I could use it with my tnc-server and GoBalloon projects.  Since your implementation calls Write() with a single byte instead of a byte slice, I added a WriteByte method.  I'm not sure if that's the best way to handle this and I'd appreciate your feedback.

My projects compile with this forked repo but unfortunately my i2c device bit the dust last night before I could properly test it.

Chris
return i2c.rc.Write(buf)