Synthesizer Interface to decouple project from SDL 2

[airloaf]

The original intention with VSynth was to make an audio synthesizer using SDL2's audio backend. It may be desirable to support other libraries in the future and have the SDL2 synth be the default implemented synth.

New Classes: Synthesizer (Abstract class that must implement the open, close, pause and unpause functions) SDL2Synthesizer (Move the current Synthesizer code into here)