number_to_words, group==2 and 3 digit numbers

The current behavior for three digit numbers with group==2 is to split between the first two and last number. For example:
```
>>> p.number_to_words('123', group=2)
'twelve, three'
>>>
```
It is more common - at least in American English - to split after the first digit which would return `'one twenty-three'`.

I realize a group is an arbitrary split, but I'm not sure the current behavior - at least for 3 digit numbers - reflects any common usage.

I realize there are a few other slightly inconvenient was to accomplish the same thing:

```
>>> ' '.join([p.number_to_words('1'), p.number_to_words('23')])
'one twenty-three'
>>> p.number_to_words('123').replace('hundred and ', '')
'one twenty-three'
>>>
```