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. i'm a total newbie with bacon so if anyone could help that would be greatly appreciated.

cheers!