Added more test cases


- Added tests for all numbers between 1 and 9999 (except for 100, 200, ..., 900), but those can be easily added later. These tests take about 52ms to run on my computer, so they are pretty fast.
- Added more tests that verify that the number replacement works when the numbers are within strings
- Added test for situations where 'a' needs to be replaced with 'one'. Ex: 'a hundred dollars'
- Reorganized the tests into their own describe blocks

One of the tests is currently failing:

```
numerizer("a thousand two hundred and fifteen")
1) Numerizer numbers in the 100 - 1000 range should treat a as 1 in some cases:
     AssertionError: "aNaN 215" == "1215"
```

Pushing up a fix for that soon.