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
it is a lot of code duplication though :( I tried it first, expanding first block on defining the "onX" methods and second block that appends those to View's callbacks, generate so much, it looked as confusing to me. Is there a better way? Like a good comment?