buffer: add Buffer.prototype.compare by offset

Adds additional `targetStart`, `targetEnd`, `sourceStart,
and `sourceEnd` arguments to `Buffer.prototype.compare`
to allow comparison of sub-ranges of two Buffers without
requiring Buffer.prototype.slice()

Fixes: https://github.com/nodejs/node/issues/521
PR-URL: https://github.com/nodejs/node/pull/5880


Reviewed-By: Trevor Norris <trev.norris@gmail.com>