create operations to get multiple ids and urls in one call

Attempts to get multiple ids and urls in one call. The result format for both these calls will be a dictionary with entries of the form ``{ url: id }``.

Notes:

* unlike ``get_string(self, id)``, ``get_string(self, id)`` does not raise ``Minifier.DoesNotExist`` when an id is not found. It just sets it to ``None``. not sure if that matters.

* I've disabled the lru cache for these calls since pylru dies on them because the argument for these calls are ``list``s, which are unhashable.

Wouldn't mind if either @kbourgoin or @tbaugis took a look