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

And this time shouldn't stall Travis CI
I know it's lame, and not a big deal, but it could improve readability if this was split up into a few functions. ie `return $this->hasValidHex() && $this->hasValidDec()`. This is one of those, not a super big deal. Just a friendly suggestion =)