.gitignore

Dear Laurent,

this PR introduces a basic `.gitignore` file which prevents `*.swp` (vim) and `*~` (emacs) files from being added to the repository.

It also removes two accidentally added emacs backup files which were introduced by the *testthat 0.8* commit 1fc0d541cbc4cd32ddd1bfe1b5565068fcaee68b.

Best wishes,

Sebastian