IntlNumberSpeller (based upon intl-NumberFormatter).

Hmm, *Intl* provides a simple way to spellout numbers by using `NumberFormatter::SPELLOUT` as `$style`.
Kay, we can reuse this already-provided way to spell numbers based on defined locale.

The current code relies in `locale_get_default` if no locale was provided on ctor, [as you can see here](https://github.com/andreyknupp/speaknumber/blob/3bd3f500ce9db7cdb6d28d588bc8cbd71a7a4954/src/Speak/Speller/IntlNumberSpeller.php#L28
).

So, we have:
- An abstraction `AbstractNumberSpeller` which allow us to reuse the validation of `$number` among the spellers;
- `IntlNumberSpeller` :beer:;
- `Number` now fallback to `IntlNumberSpeller` as default if `extension_loaded('intl')`.

That's all folks.
:beers: 

- `NumberFormatter` http://au2.php.net/manual/en/class.numberformatter.php;
- [locale_set_default](php.net/locale_set_default) and [locale_get_default](http://php.net/locale_get_default).