Fixes #72 - Restructured arb int math and enabled BCMath as an engine

And this time shouldn't stall Travis CI
I would personally like to see this interface include more documentation. As stupid as it sounds `add($a, $b)` is it $a + $b or $b + $a?

The other thing I want to see is the `cmp` function documented. Oh and also `@return` on the functions.

Have you considered adding the `input()` function in here as well?

All classes that implement this interface you can use the `@inheritdoc` to tell developers to look at the interface.