remove server code

Original patch by Danny Coates, extracted from PR #9.
Merge pull request #14 from warner/remove-server

remove server code
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.
make public interface take Buffers, not numbers

Original patch from Danny Coates, extracted from PR #9.

This puts a converting wrapper around the internal functions. A later
refactoring will move this conversion down into the functions and get
rid of the wrapper.
srp.js: update docstrings
Merge pull request #17 from warner/buffers

make public interface take Buffers, not numbers