some test fixes

changed package.json to use `jest-cli` and not `jest`
added jest-related configs in `package.json` that will circumvent the "function" error you were seeing, but will yield a different error.
added a test that allows you to completely mock the `body-parser` module.
Interesting, I see that you're asking `Jest` to not mock `body-parser` but to mock its `json`.  That's great stuff.  I have no idea how I'd be able to work this out without your help. :)  I wonder if there's anything you can do to the `jest` code to be able to avoid the need for all this?