to.have.length() for non-zero length

I would expect `length()` to be able to check for _any_ non-zero-length. The cleanest API for this, to me, would be passing no argument:

```
expect(filled_array).to.have.length()
```

Also, more powerfully, it'd be nice to be able to chain the numeric operators onto this:

```
expect(filled_array).to.have.length.greaterThan(2)
```