Use github/gitignore Leiningen.gitignore

Remove a little friction from development by .gitignore-ing all Leiningen artifacts, by copying over Github's gitignore file for Leiningen projects: https://github.com/github/gitignore/blob/master/Leiningen.gitignore.

The Github file is a superset of the currently ignored files, except it doesn't account for emacs swap files `*~`. This should be preserved in the update .gitignore.
