Use grunt-sync to only copy static files when they are modified

This pr uses `grunt-sync` to only copy static files that have changed. This significantly improves my dev experience when working with sails because I spent much less time waiting for files to be copied to the .tmp directory after making a change to my client side js files.

This pr was in response to issue #719 I also suspect it will help when using bower like in #1122.

Unfortunately `.less` `.jst` `.coffee` files are still copied and compiled every time. I'm not using may of these files formats currently so I have not noticed a problem but I suspect coffeescript projects will still experience a significant delay while `.coffee` files are compiling.