Ignore directory changes

After a `git checkout` it was common to see things like this from the server:
```
ERR! ol Trouble reading script.  Expected a file name, got a directory name instead: /path/to/ol3/src/ol
```
This was caused by gaze emitting `add` events for directories even if they didn't match the watch patterns.  This should be resolved when `v0.5` is published.  This workaround solves the problem until then (and may be a long term fix for Node 0.8 support).