Fix some minor UX issues with cake test

This just adds some things that annoyed me when first using `cake test`:
* State that you need nodeunit when running the tests and in `README.md`
* Add a `--verbose` option to show the test output
* Tell the user if the tests succeeded or not (`error` != `0`)

These are all pretty basic changes but they should save people who just want the run the tests quickly a few minutes of pain :)