Make the watcher more responsive to child changes

the watcher for files with lots of imports. The regression was caused
by calling the slow `gaze.add` unnecessarily. We only need to call
`gaze.add` on files that aren't currently being watched.

At the time I confirmed that calling `gaze.add` in files that were
being watched wouldn't result in a leak or multiple events being
fired. I however assumed calling `gaze.add` for already watched
files would be very cheap.

Fixes sass#1869

bin/node-sass

@@ -268,7 +268,10 @@ function watch(options, emitter) {
 
     // Add children to watcher
     graph.visitDescendents(file, function(child) {
-      gaze.add(child);
+      if (watch.indexOf(child) == -1) {
+        watch.push(child);
+        gaze.add(child);
+      }
     });
     files.forEach(function(file) {
       if (path.basename(file)[0] !== '_') {