Add filterResources method so that filtering only happens once per render

The fetchResources function gets called in a loop every time the calendar renders. The resourceFilter function is called one time for every resource each time the fetchResources function is called.

Here's a plunkr demonstrating the issue: http://plnkr.co/edit/hldlvdcgRSJgvPpKVSIp?p=preview. The counter is reset every time the view renders.

This is also an issue in @seankenny's fork: https://github.com/seankenny/fullcalendar/issues/45 

In this particular example, the function is being called up to 24 times, despite there only being 3 resources. Not a big deal. But, when I was using about 20 resources the function was being called up to 860 times on every render, which brought the UI to a complete freeze for a few seconds.

__This PR is incomplete.__ I haven't yet found a way to call `filterResources` on render. Maybe you know a way?