Removed report as an option; Return raw css string in a callback.

Sorry for the confusion.

There has been [some concerns](https://github.com/addyosmani/grunt-uncss/pull/54) regarding whether `report` option should return computed file sizes of a input and output.
It has been proposed that uncss should return raw css string to be let for manipulation later on.

Please review the changes.

Best,
Robert.