Abstracting Boolean Value in read/write strings

Implementation of the changes discussed in Issue #20 

Implemented _get_bool_str(value), but had to put a version in both Base and OCP for power supply.  I didn't readily see a good place to put this only once.  

I modified the Rigol base object to eliminate the unneeded methods.

This needs tested with 1/0 style power supplies to make sure original functionality isn't broken.  I don't have access to anything with that protocol.