Fix earlier commit for grunt watch.

refs TryGhost/Ghost-UI#18
- Trigger livereload when files in core/client/assets/css/ change
- Copy over new ghost-ui files when changed


This fixes my earlier commit where `grunt watch` was triggered but didn't update ghost-ui files.

**Note:** It would be great if someone could look over this. The `'ghost-ui'` subtask is triggered twice when a change in ghost-ui occurs. This is due to two files being changes. There is a `debounceDelay` option which however doesn't seem to prevent triggering the task twice. I looked into `_.debounce` options, but those are overly complex for the simple use case we have here. 

TL;DR;: This PR fixes the issue from the earlier commit but triggers the copy:dev task twice.