Added more-legitimate tests + code coverage

I added a better test suite that's more automated than having to read console.log's to see if something is or isn't working. The tests can be run with either of the following:
```sh
$ make test
$ npm test
```
I also added configuration for running them automatically on TravisCI (so that future PR's will report test results + coverage). For this, you'll need to sign up and enable tinycache on [TravisCI](https://travis-ci.org/) and [Coveralls.io](https://coveralls.io) (both are free). For a demo, checkout the [naturalatlas fork](https://github.com/naturalatlas/tinycache) of tinycache.

This PR also includes a bugfix for `misses()` – it wasn't reporting misses caused by non-existent keys.