Support unrolled arguments to ajax-request

As discussed in #29.

The new usage has named arguments unrolled instead of in an explicit map, and uses `:manager` as the name of the previous unnamed, optional fourth argument. Old usage is still supported and tested.

I didn't unroll the maps passed to `GET`, `POST` and `PUT`. I can do that as a different PR, or go back and add it to this one if you would like.