Add proper testing

Remove old testing function, it wasn't really testing anything.

Added initial simple tests using Mocha.

```sh
# using mocha directly
$ mocha

# npm test script
$ npm test
```

```sh
  moonmoji
    no date object argument
      ✓ should return the moon for today
    date object argument
      ✓ should return correct for waxing crescent
      ✓ should return correct for waxing gibbous
      ✓ should return correct for waning crescent


  4 passing (10ms)
```
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You should add moonmoji to [Travis-CI](https://travis-ci.org/) after, and add the badge to the `README`.