Refs #24 (Add asnyc validation)
Refs #28 (Support async validation in Configuration and Settings objects)
apllied eslint
updated Assertion/InstanceOf and Assertion/Schema to support async validation
updated Assertion/Exact to support async validation
updated Assertion/Custom to support async validation
updated Assertion/Empty to support async validation
updated Number type assertion to support async validation
updated ChildOf assertion to support async validation
updated String assertion to support async validation
updated Optional assertion to support async validation
updated Number/Integer assertion to support async validation
updated Object type assertion to support async validation
added tests for Object#property assertion
updated Array assertion to support async validation
applied eslint
restored Testable#normalize method
removed Reference element
removed Testable#normalize method
replaced .length with .count() calls
updated gulpfile