Allow user to not specify a destination in Gruntfile

My changes allow the user to not specify a destination or specify the same destination as the source and it will produce the expected output. This allowed me to chain the cssmin task after other tasks that affect the same CSS file (grunt-contrib-less and grunt-combine-media-queries).
![no_destination](https://f.cloud.github.com/assets/4976096/895733/884e4686-fad2-11e2-89cd-af8bb9d7cbed.png)