Feature/tests coverage

@calebeoliveira I've added some unit tests in the project. Please let me know if you have any questions.

Running
```shell
$ composer update
$ php vendor/bin/phpunit --bootstrap vendor/autoload.php tests/
```
Output:
```shell
PHPUnit 4.2.2 by Sebastian Bergmann.

....

Time: 32 ms, Memory: 3.25Mb

OK (4 tests, 6 assertions)
```

Take care, Jonata Weber