Allowing control of long stack support through Q_DEBUG environment variable

This is a feature present in Bluebird that is extremely useful for development purposes. Depending on project directory structure and use of "npm link," several different instances of Q can be referenced in the same runtime. In this scenario it can be difficult to hunt through different files across different modules just to find the right places to put a "Q.longStackSupport = true" line in order to get a reasonable stack trace. Enabling it globally through an environment variable gives a simpler solution.