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

And this time shouldn't stall Travis CI
You could delete this statement if you move it to the first if. ie `if (bccomp($a, $b) == -1 || bccomp($a, $b) == 0)` or you could even change it to `if (bccomp($a, $b) <= 0)`

Seems that they both do the same.