Move to grunt 0.4; add banner.

I can make this two different PRs if you prefer.

The first commit makes the move over to Grunt 0.4. The only code change necessary was to remove the helper which does nothing (Grunt 0.4 does not have the registerHelper function). I also added a test which makes sure snockets is concatenating files correctly.

The second commit adds an optional "banner" argument which allows us to add arbitrary text to the top of a concatenated file. This is useful, for instance, to add a warning to the destination file that the file is generated and should not be edited by hand.

Solves #1