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
I also think we shouldn't hard-code the fact that when we get a view callback like onViewReady, we translate it directly into a template callback like onRendered.  It's semi-coincidental that implementing template callbacks is so easy.  At one point in time, for example, implementing onRendered meant listening to a view callback and then calling `Tracker.afterFlush`.  So there's no fundamental reason why each template callback is implemented in terms of one view callback with no wrapping or adapting.