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
Look in `view.js` for `Blaze._fireCallbacks` to see how `View` stores its callbacks.  You can use a similar pattern.

So instead of `self.rendered.call(view.templateInstance())` on line 139, it would be something like `self._fireCallbacks(view, "rendered")`.