buffer: add indexOf() method

Add Buffer#indexOf(). Support strings, numbers and other Buffers. Also
included docs and tests.

Special thanks to Sam Rijs <srijs@airpost.net> for first proposing this
change.

PR-URL: https://github.com/iojs/io.js/pull/561


Reviewed-by: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>