src: add BE support to StringBytes::Encode()

Versions of Node.js after v0.12 have relocated byte-swapping away from
the StringBytes::Encode function, thereby causing a nan test (which
accesses this function directly) to fail on big-endian machines.

This change re-introduces byte swapping in StringBytes::Encode,
done via a call to a function in util-inl. Another change in
NodeBuffer::StringSlice was necessary to avoid double byte swapping
in big-endian function calls to StringSlice.

PR-URL: https://github.com/nodejs/node/pull/3410


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>