#1670 - :default now expects proc for index scopes

Resolves #1670, allows you to pass a proc to the `:default` option for a proc.  For code cleanliness, method now expects a proc (much like Rails 4 does with scopes anyway).

The only thing I'm thinking of is finding a way to execute the proc in a view context, so that you can use CanCan for instance, which doesn't work right now and is what most people will want to use this for I imagine.

Thoughts @Daxter?