support custom bindings, fixes #37
Merge branch 'custom_binding' of https://github.com/Dashwire/representable into binding
require 'mocha' again to make it run with all mocha versions.
introduce a generic Binding::build_for and call it in all subclasses.
:binding now accepts a lambda that is responsible for creating the proper Binding instance.
remove Definition#has_binding? in favor of #binding.
remove mock tests.
added :binding documentation in the README.