Handle destination directory being /.

When the destination directory is `/` and no files were specified
(because we want all files), an error was thrown because we were
attempting to symlink the already existing temp directory (which was
created by `quick-temp` in `broccoli-writer`).

* Add basic tests illustrating the issue.
* Add JSHint.
* Create basic test for file globbing (to ensure no regressions from
  this change).

@linstula & @rwjblue
Merge pull request #14 from linstula/destDir-fix

Handle destination directory being /.