gitignore: ignore xcode workspaces and projects

Calling ./configure --xcode creates xcode projects and a
workspace for io.js.

PR-URL: https://github.com/iojs/io.js/pull/1562


Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Reiss <me@silverwind.io>