Add LooseSignatureVerifier

See https://github.com/rspec/rspec-mocks/pull/726
I think this pattern (i.e. raising an error in the base class to indicate subclasses must implement) is nice and useful when the class is intended to be subclassed by users of the library, but the subclasses are all defined inline in this file, and this doesn't seem useful or necessary.  While I'm sure it's not noticable, it does take some time for ruby to define a method at boot time, and with enough defined-but-unused method definitions it could start to get noticable...so I vote we remove this method def.