Song metadata: Added date and author

Big but necessary change to database. I have also written a small snippet that allows to convert the "old style playlists" to the new type. Trying to open an old playlist results in a white screen, because the changed schema basically fails to validate ("songs" property has changed from a [String] to a [Object]).

I've tested the following snippet; it shouldn't give any problems:

```
Playlists.find({}).fetch().forEach(function(playlist) {
	if(Match.test(playlist.songs,[String])) { //if playlist is "old"
		console.log("old playlist");
		
		var newSongObject =[];
		playlist.songs.forEach(function(song) {
			var songObject = {songId:song, added:new Date(), author:""};
			newSongObject.push(songObject);
		});
		
		console.log(newSongObject);

		Playlists.update({_id:playlist._id},{$set: {songs:newSongObject}});
	}
});
```

It is probably best to run the code on startup on the server once, to make sure all playlists get updated, not just the ones published to the client. :wink: 

(Fixes #43 )