Move to app/assets in anticipation of image support being added as well. Also add vendor/assets to default load path

After a long list of discussion about the performance problem from using varargs and the reason that we can't find a great pair for it, it would be best to remove support for it for now.

It will come back if we can find a good pair for it. For now, Bon Voyage, `#among?`.

I don't know which is the use case for button_to_function(name)
but there's a test for it. I am focused now on RJS extraction
and do not want to introduce a backwards incompatible change
at this moment. Perhaps worth revisiting when the whole thing
is done.

Deprecate defining scopes with a callable (lambda, proc, etc) via the scope class method. Just define a class method yourself instead.

Evaluate default scopes at the last possible moment in order to avoid problems with default scopes getting included into other scopes and then being unable to remove the default part via unscoped.

Deprecated support for passing hashes and relations to default_scope, in favour of defining a 'default_scope' class method in the model. See the CHANGELOG for more details.

Removing the scope-caching which happens on association proxies, because the query is already cached by the query cacher. For formalised proof see http://www.youtube.com/watch?v=wDefXLb-FDs