Added optional callback methods for tiral start and trial finish events in core library.

Can now pass in functions to the core library on init() so that a
function can be called at the start or end of every trial.

jspsych.js

@@ -25,14 +25,22 @@
 		var initialized = false;
 		// target DOM element
 		var DOM_target;
+		// trial start callback method
+		var on_trial_start;
+		// trial finish callback method
+		var on_trial_finish;
 		
 		//
 		// public methods
 		//
 		
 		core.init = function($this, options){
+			var defaults = {
+				'on_trial_start': function(){ return undefined; }
+				'on_trial_finish': function() { return undefined; }
+			}
 			// import options
-			opts = $.extend({}, jsPsych.defaults, options);
+			opts = $.extend({}, defaults, options);
 			// set target
 			DOM_target = $this;
 			
@@ -147,7 +155,14 @@
 		
 		function do_trial(block, trial)
 		{
+			// call on_trial_start()
+			on_trial_start();
+		
+			// execute trial method
 			jsPsych[trial.type]["trial"].call(this, DOM_target, block, trial, 1);
+			
+			// call on_trial_finish()
+			on_trial_finish();
 		}
 		
 		return core;