install: install iojs -> node compat symlink

Install a symlink for compatibility with scripts that expect a binary
called `node` or have a /path/to/node shebang.  Only done on UNIX
platforms because symlinks on Windows require elevated privileges.

PR-URL: https://github.com/iojs/io.js/pull/262


Reviewed-By: Bert Belder <bertbelder@gmail.com>