Added exitChain function. ExitChain function stop the execution of a Step chain of callbacks.
Changed the parallel method of Step. Parallel now accepts arguments to be appended to the cb response. Second modification is that all the arguments from the callback response are sent to the next step chain instead of just the first.
Merge branch 'master' into develop

Conflicts:
	lib/step.js
	test/exitChain.js
Added option to exit step chain once an exception is encountered. Also added another option to register a function to run whenever an exception is triggered.