Augment math code to include BC and native arb int math code so we can support the widest PHP install base

[ionux]

GMP classes ported to BC. Does not include native implementation yet. Library will need to be modified a bit to more generically handle math operations and pass off to correct math class, i.e. strategy pattern implemented.

[ionux]

https://github.com/ionux/php-bitpay-client/blob/master/src/Bitpay/Util/Bc.php

[aleitner]

#104
Merged