Progress towards old {{> yield}} and {{#contentFor}} semantics

(Given that 0.8.0-rc3 changed the lookup rules to not look up
the component chain)
Apply healthy dose of duck tape.

- cc @tmeasday: Can you test this out in your app and see if things start
  breaking? Tests are passing, and things seem okay in a simple demo app. I
  wasn't able to test in EM because of other updates I need to make to work with
  the latest 0.8.0-rc3.

- Special thanks to @avital from Core for helping with this issue!

- Hopefully fixes [Issue 553](https://github.com/EventedMind/iron-router/issues/533)