Added a Sound Data block

This block returns a number between 0 and the square root of 2 (about 1.414).  It accesses the SoundMixer.computeSpectrum() method to get the raw sound data of all currently playing sounds.

![capture](https://cloud.githubusercontent.com/assets/7967463/3617349/9472328c-0ddd-11e4-87d0-1d2c8d00313e.PNG)

It can be used to create enemies in a game based on the frequency of the music playing, to create audio visualizers, etc.

A project where this could be implemented is here: http://scratch.mit.edu/projects/24655725/.

If you would like to test it online, here is the .swf file: https://dl.dropboxusercontent.com/u/76236043/Scratch%20-%20Sound%20Data.swf

Here is a song that works great for testing!: https://www.dropbox.com/s/fmdjapbkr3ak368/Transcendence.wav