Added code minifier using grunt as dependency

This may be totally unnecessary as part of the repo and I just did it in my cloned copy of the repo, so I could serve the minified version of jPlayer right away. 

The only reason I considered submitting a pull request was that in relation to [this thread](https://groups.google.com/forum/#!topic/jplayer/fJwtdOxJ8nA) where users were unsure how to make the change to the source code, I figured they might also be wondering how to minify the source and what to do next (I was anticipating that question on the board, actually). 

Anyway, if they have `npm` installed, they can just clone the repo, then do:

    npm install
    grunt uglify

On the plus side, it's an easy way to get people the minified code without having to minify it, and `grunt` can be used for other stuff.

The big problem, though, is that if users don't know how to minify js source *and* they don't have `npm` installed, this won't be that useful.

Thus, I thought this might be helpful for inexperienced users cloning the repo, but feel free to reject this pull request if it doesn't fit in with jPlayer as a whole. 

Changes:
Added: gruntfile.js, which includes command to minify jplayer source
Edited: package.json to include 'grunt' and 'grunt-uglify' as dependencies.