Added log verbosity methods (V, SetVerbosity, and IsV) to factorlog.Logger interface
Added those functions to factorlog.NullLogger

This allows you to accept a Logger and pass it either a FactorLog or NullLogger object.
Implemented SetVerbosity, V, and IsV as methods of the Verbose struct

Added test function for these
Merge pull request #4 from jmunson/master

Added verbosity methods to Logger interface, made NullLogger implement them