add R.apply

I'm surprised this function isn't yet defined. It's similar to [clojure.core/apply][1], the difference being that Clojure's apply is variadic whereas Ramda's is a curried binary function.


[1]: http://clojuredocs.org/clojure.core/apply

Most of our other function-generating functions keep the context.  Any reason not to do

        return fn.apply(this, args);

?