Merge if you like math!

GMP is no longer a dependency. If GMP does not exist, BC Math is used instead.
Why not have these tests

```php
$this->assertEquals($math->add(1, 1), 2);
```

Also https://phpunit.de/manual/current/en/appendixes.assertions.html#appendixes.assertions.assertEquals says that the arguments should be switched, but it's fine as is right now.