Audio: Add Decoder#canSeek, setPosition and getPosition.

- Adds seeking (getPosition/setPosition) to Decoder
- 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