Allow helpers to work inside `{{#Layout`s.

Ugggh. That was not an easy monkey patch.
 
@cmather - I don't know if we should ship this but it appears to work the way I wanted. It's much easier to just patch Meteor and change ui/base.js:findComponentWithHelper directly.

In any case, you'll see that both helpers are now working as they should. 

The basic change to blaze is that you can define `__passthrough` on a component which means it both acts as a helper host (in that it provides helpers) and not as one (in that it can be passed-through for look up).

Apart from philosophical objections, the one thing that concerns me is that helpers defined on the layout template will work within the yielded block. This will be surprising to users, but I can't see a way around it without doing something crazy like setting the `.parent` on the `yield` component to the outer template.

P.S. potentially this could mean you could get rid of your other hack (for `yield` and `layout` and instead set `__helperHost` and `__passthrough` on the `Layout` component itself).
Get rid of spurious logs
Add some comments for the new __passthrough property.
Bump version to 0.2.4.
Fix template method to check arg length to determine getter/setter.

If you call myLayout.template(undefined) it should treat this as an assignment,
not a get.

cc https://github.com/EventedMind/iron-router/pull/607.
Add test for setting undefined layout.
Use deps-ext Deps.cache instead of UI.emboxValue.
Remove console.log.
Bump version to 0.2.6.
Revert "Add some comments for the new __passthrough property."

This reverts commit 42db5f574ca692b8a03f492c5ee0624f60a3fb51.
Revert "Get rid of spurious logs"

This reverts commit 6ecda8e0493c7b0034bc3618fa09a3ec3ce5dfff.
Revert "Allow helpers to work inside `{{#Layout`s."

This reverts commit 742448ddff7f05ff02a7d49082b4667d5ecf2998.
Got rid of `__helperHost` hack and use `UI.InTemplateScope` instead.

This is the "correct" way to do it, and the way that Spacebars does it for template includes and `UI.contentBlock`
Added test + use UI.InTemplateScope.