added HITLayoutId as options for creating HIT

Added HITLayouts and HITLayoutParameters to createHIT(). Also updated README with corresponding information.


Layout made on requester page, used to test:
&lt;h1&gt;${param1}&lt;/h1&gt;
&lt;input name="${param2}" required="" type="text" /&gt;

Corresponding function call:
mturk.RegisterHITType(RegisterHITTypeOptions, function(err, HITTypeId){
    if (err) throw err;
    console.log("Registered HIT type "+HITTypeId);
	    var CreateHITOptions = {
	    'HITTypeId': HITTypeId
	    , 'HITLayoutId': '2UYP8PJWKUDJBZ4G4VU920B6UVFJ8X'
	    , 'LifetimeInSeconds': 60 * 20   
	    , 'HITLayoutParameters': {
	    	param1:'foo',
	    	param2:'bar'
	    }
	};
	mturk.CreateHIT(CreateHITOptions, function(err, HITId){
	    if (err) throw err;
	    console.log("Created HIT "+HITId);
	});
});