Allow resources to be set via function

Allows the `resource` option to be set in the constructor as a function. The required return value **must** be an array.
```
	resources: function() {
		return ['resource 1', 'resource 2'];
	},
```
```
	resources: function() {
		$.ajax(...)
		.success( function(data) {
			// some sort of logic
			return preppedData;
		});
	},
```
This should solve [Issue #8](https://github.com/AbleTech/fullcalendar/issues/8).