Ignore IntelliJ files

[iconara]

I second @bittrance's comment for the general case, it's better if people use their global ~/.gitignore or repo-private config to ignore their own favourite combination of files, otherwise we'll end up with the union of all tools' files in every single repo.

However, since we already ignore Eclipse as @bittrance says, and I seem to have a commit that I just hadn't pushed that ignores /.idea I think this makes sense to merge 😳

[slandelle]

Thanks