Switch from bigint to bignum.

This makes node-srp easier to use (simplified dependencies). It is about
30% slower (getV with bigint took 1.78ms, bignum took 2.35ms, on my 2012
MBP laptop). Run test/speed.js to measure this.

Original patch from Danny Coates, extracted from PR #9.
Merge pull request #16 from warner/bignum

Switch from bigint to bignum.