Add normalize parameter to synthesize and fluidsynth methods

[yawjalik]

Added normalize parameter such that when set to True, hard-normalizes the synthesized output wave to [-1, 1]. Otherwise, divide the wave by len(self.instruments) * 2^15 to prevent clipping, and lets the user handle accordingly. It is set to False by default. This serves as a starting point for tackling #151 and #158.

[yawjalik]

This is probably my first ever contribution. Any comments or advice will be greatly appreciated!

[craffel]

Thanks for this. Since the original behavior was equivalent to normalize=True, that should probably be the default to avoid breaking existing code.

[yawjalik]

Got it, I've changed them to True by default.

[craffel]

Thanks!