Fix nested async exits and strict-mode handling

• Correctly handle async return|throw extension if nested more than one function level deep (see tests/semantics/nested-async-exits.js)
• Provide default value for engine in configuration object so it can be programmatically over-ridden
• Enforce use strict in tests unless --notStrict is specified
• Correct handling of strict mode nested function hoisting to mimic the semantics of recent browsers
• Run tests against NodeJS 0.10, 0.12, 4, 5, 6 and 7 in both strict and sloppy modes