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

And this time shouldn't stall Travis CI
I'll certainly add more documentation
@JoshuaEstes : the input function is only needed by BCMath engine right now, and any other engines we implement may use something similar or nothing at all like the GMP engine.  As such, it's not necessary for it to be in the interface, but maybe should me made private again or annotated explicitly as not part of the API in the engine itself.