[#16] Rename MethodSignatureValue#accepts? to #belongs_to?
[#16] Rename MethodSignatureValue#value to #content.
[#16] Rename MethodSignatureValue#modified_class to #content_class.
[#16] Rename MethodSignatureValue to MethodSignature::Value.
[#16] Rename Value#as_instance to #content_as_instance.
[#16] Introduce factory-like Value.from(content).
[#16] Move boolean value handling to MethodSignature::BooleanValue.
[#16] Move class value handling to MethodSignature::ClassValue.
[#16] Move rspec double handling to MethodSignature::RspecDoubleValue.
[#16] Fix: 'active_support/core_ext/string' should be required in lib/verified_double.
  Make unit_helper and spec_helper synonymous for now. Test suite is still fast even with all the requirements.
[#16] Move recording double class value handling to MethodSignature::RecordingDoubleClassValue.
  Add RecordingDouble#doubled_class.
  Clean up RecordingDouble spec.
[#16] Move instance double value handling to MethodSignature::InstanceDoubleValue.
  Let RspecDoubleValue inherit from InstanceDoubleValue.
[#16] Move instance value handling to MethodSignature::InstanceValue.
  Let RecordingDoubleClassValue inherit from ClassValue.
  Let InstanceDoubleValue inherit from InstanceValue.
  Let BooleanValue inherit from InstanceValue.
[#16] Drop unit_helper for now.