Support audio playing

A few natives were implemented to support playing wav and amr format audio file streams. 

I'm not sure how to write unit tests for audio playing and added a sample midlet from nokia document to test manually.

The amr audio format is not natively supported by browser. I had to include an amr codec (http://jpemartins.github.com/amr.js/) temporarily. One issue is that the codec doesn't provide any license file. I added a REAME.md file to explain the code source, but don't know if it is enough to address the issue.

The codec is ported from opencore-amr under apache license with emscripten tool. I'm going to port one to fully resolve the license issue in the future...

Finally, it fixes #534.
Nit: I would use an Object or Map for this cache. Then you don't need the complex code to reuse IDs and find the next free one. You can simply increment *_nextId* each time you need a unique one.