Background jobs

For different reasons, we need to run things in the background:

* Change SSH keys
* Delete branches
* *later* Edit a file
* *later* Clone a remote

For this reason, I bootstraped a simple bundle to handle background jobs.

Many things must still be addressed:

- [ ] Signal handling in shell
- [ ] Test branch delete feature
- [ ] Implement SSH Key as background job
- [ ] Technical documentation for background jobs