Some improvements were made.

I'm against this title, but...

As I've commented at #2, we shouldn't care about how big the number is (yep, there's a limit, but not a small one.).

P.S: the *limit* to "speak" now is `PHP_INT_MAX`.
> PHP_INT_MAX (integer)
The largest integer supported in this build of PHP. Usually int(2147483647). Available since PHP 5.0.5
&mdash; http://php.net/manual/en/reserved.constants.php

O.K, this PR proposes:
- An abstraction of how number spelling is done (we can give a speller to `Speak\Number`) or even set it by using `setSpeller` (`SpellerAwareInterface`, `SpellerAwareTrait`);
- A interface which allows us to implement the spelling for other languages &ndash; `NumberSpellingInterface`;
- Making the instance of `Speak\Number` reusable to spell others numbers in runtime;
- Adding exceptions `NumberIsTooLargeException` and `NegativeNotAllowedException` to be more specific on what happened wrongly when a exception is thrown at `Speak\\Number()->speak(...)`.
- Adding the tests for all of these monkeyshines.

That's all for now.
:beers: