scan_for_focus_path should return an empty array if path is a directory

![Screen Shot 2013-01-04 at 1 52 46 PM](https://f.cloud.github.com/assets/420959/44050/7910b646-56a8-11e2-9fcb-1d00b76161b4.png)


When I have set the `focus_on` option, and I run all feature tests in cucumber-guard, then a directory path may be sent to `Focuser#scan_for_focus_path`. This method tries to call `File#gets` on the directory, and ruby throws a "`Is a directory`" error.

This patch simply returns an empty array if the file is a directory.