Add .gitignore

If you are not included in the repository xcuserdata is better.

https://github.com/github/gitignore/blob/master/Swift.gitignore

> $ git submodule add git@github.com:aleclarson/emitter-kit.git
> open Xcode
> $ git status
>    modified:   emitter-kit (untracked content)
> :/
> $ cd emitter-kit
> $ git status
> 
> Untracked files:
>   (use "git add <file>..." to include in what will be committed)
> 
>         EmitterKit.xcodeproj/xcuserdata/{my_user_name}.xcuserdatad/
> 
> :/