Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
This mocking seems a bit brittle, too....it's coupled to our use of `public_method_defined?` elsewhere which I consider a bit of an implementation detail. I think it would be better to directly define a `customise!` method on the the test class such that it defines a new method.  If you're worried about that being a global mutation (I'm guessing that's why you opted for the mocking here...), consider defining a one-off `Class.new` and using that instead.  Or you could keep the existing class but use a one-off subclass so that only the subclass is mutated and then discarded.

It might also be worth naming `customise!` `define_attribute_methods` to more clearly show the case that prompted this.