Add approximate and approximately

Approximates the `to.approximate` and `to.be.approximately` syntax from `should.js`.

```js
expect(0.99999).to.approximate(0.99998, 4); // works
expect(0.99999).to.approximate(0.99998, 5); // fails
expect(0.999999).to.approximate(0.999998, 5); // works
expect(0.999999).to.approximate(0.999998, 6); // fails

expect(0.99999).to.be.approximately(0.99998, 4); // works
expect(0.99999).to.be.approximately(0.99998, 5); // fails
expect(0.999999).to.be.approximately(0.999998, 5); // works
expect(0.999999).to.be.approximately(0.999998, 6); // fails
```

This closes #83.