Add shuffle mode to Jam

[hwray]

Users want the ability to set the Jam to a "shuffle" mode. Some steps this will require:

• Add a shuffle() method to the Jam, which randomizes the indices of all the songs after the song that is currently playing in the Jam.
• Add a global data structure that can store the old indices of each song in the Jam, so that users can toggle in and out of shuffle mode to go back to the original song orderings.
• Add an isShuffled global boolean, which we check in addSong() to determine whether to insert the new song at the end of the list (normal mode) or at a random index (shuffle mode).

[avptv]

I'll make some shuffle vs loop icons to use as the shuffle button

[hwray]

I hate to sound like a broken record, but 'd like to stress yet again that our time is best spent not making custom buttons. Unless you have a really good idea and genuinely think you can design a shuffle logo that is more simple, iconic, intuitive, and easily-recognizable to users than any of these free ones:

http://www.flaticon.com/search/shuffle

We didn't get any feedback indicating users wanted a loop functionality, so I'm not going to add that for now.

[avptv]

I'm not at all saying I'm going to reinvent the look of a shuffle logo. We just need one that goes with the rest of our buttons. I was just going to make a simple shuffle logo in our red, like I did with the play/pause buttons. Our red is #FF0927 so just finding a shuffle logo in red probably isn't going to match. I think making it all match and look good is very important.

As for the loop, the loop button would just be to get out of shuffle mode, to go straight through songs instead of shuffling