Add delete modifier and update README to reflect API

The README shows a delete modifier being used, but one doesn't seem to exist in `oj.modifiers`. This patch adds a `delete` function and updates the README. 

Note that the README demonstrated a signature that accepted (in addition to a query map) a boolean value. I can't think of an example where you'd want to pass `:delete false`, so I removed the second parameter and hard-coded `true`.

I also added a couple of tests for the modifiers stuff.

Take care, and keep up the good work!

Erin