compatibility with java.math.BigInteger

add, subtract, multiply, divide, remainder, negate, compareTo are implemented by all those libs:
https://github.com/dtrebbien/BigDecimal.js
https://github.com/silentmatt/javascript-biginteger
https://github.com/iriscouch/bigdecimal.js
http://www-cs-students.stanford.edu/~tjw/jsbn/