Add a gitignore file!

This will make sure that any cached files aren't persisted across
cloning of this repo. The best way to generate one of these files is to
grab the necessary stuff from GitHub's gitignores repo at
https://github.com/github/gitignore
Delete any files we don't want tracked.

These are being ignored by our gitignore file, and they're safely
regenerated by Xcode each time it opens (if they don't exist).