Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
`"#{test_class.inspect}"` is a little odd -- `test_class.inspect` is already a string and then you're interpolating it into a string with no other content.  And actually, there's no need to use the string form -- `test_class` will work just fine (and would be the only way to make this work if you use a one-off class that hasn't been assigned to a constant).

Also, you used 1.9 hash syntax, which'll break the build.