Audio: Add Decoder#canSeek, setPosition, getPosition, readSamples, CircularFloatBuffer, etc.

- Adds seeking (getPosition/setPosition) to Decoder
- Add Decoder#readSamples(float[]), (it makes sense to let the decoder decide how it reads its samples, just like AudioDevice has a writeSamples(float[]))
- Add CircularBuffer#skip and #getAvailable.
- Modify CircularBuffer#combine (it was based on unsigned short, see http://www.vttoth.com/CMS/index.php/technical-notes/68, I might add a better combining method).
- Add CircularFloatBuffer, which is basically a copy of CircularBuffer but with float[] instead of short[].  
- Implements seeking in VorbisDecoder

I'll add seeking later on for Mpg123Decoder and WavDecoder. Currently testing a easier to use API for audio DSP using gdx-audio, which I will commit in parts (if you like), this is the first. I'm also considering updating some libs (like soundtouch) with the latest version.

Simple example, showing some basic usage: http://youtu.be/ih35EqKZRSw