Allow watch on multiple directories rather than a single directory

Hi,
Made a one line change to allow multiple folders to be watched rather than a single one.

i.e. in gruntfile.js
                watch: ['app','lib','typings'],                  // If specified, watches this directory for changes, and re-runs 

seems to work quite nicely for my purposes, but I'm new to grunt, node and git so it might have unexpected consequences!