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.
sure, no problem. just so you can see the existing issue (again, not sure if it's the same issue concerning `fs` module) https://github.com/facebook/jest/issues/106