add async tests for Model.apply

hi! thanks for `bacon.model`. :)

i wrote some tests for `Model.apply` that demonstrate using async streams.

the reason i wrote these tests is because i'm having an issue using `bacon.model` in a [`bacon.level`](https://github.com/holonomy/bacon.level) plugin i began working on. i'm clearly doing something wrong as this plugin's tests pass and my plugin's tests fail, but i'm having trouble figuring out what is wrong. if anyone could help that would be greatly appreciated.

cheers!