Bootstrap packages added to load path

This line https://github.com/rejeep/cask.el/blob/master/cask-cli.el#L31 will add the bootstrap packages to `load-path`.

To reproduce:

```bash
$ mkdir foo
$ cd foo
$ cask load-path
/Users/rejeep/.emacs.d/.cask/24.3.1/bootstrap/commander-20130812.2237:/Users/rejeep/.emacs.d/.cask/24.3.1/bootstrap/f-20130729.1445...
```

@lunaryorn You know this best. I guess this is expected behavior for `epl-initialize`, so maybe we should wrap the `unwind-protect` with something like this:

```lisp
(let ((load-path (copy-seq load-path)))
  (unwind-protect
      ...
    ))
```