Add onEvent methods to Template

Add `Template#onCreated`, `Template#onRendered` and `Template#onDestroyed` methods. These are better than assigning `Template.rendered`, etc properties as you can add more than 1 of them w/o overriding previously added event handlers.

Ran test-packages for Blaze.

@avital @dgreensp
As a matter of style, let's not use this "compressed" way of specifying multiple functions; let's just write them out.  It's easier to read that way.