Update dsd.h, dsd_audio.c, dsd_main.c and dsd_symbol.c

Adding portaudio support. Implementing it as a new audio_..._type in dsd.h.

Update CMakeLists.txt and create FindLibPortAudio.cmake

CMake portaudio check
Update dsd_audio.c

Small oops
Tweaking the delay a bit... 0,050 s for output seems too little; it results in choppy audio and ultimately, a heap of errors. 0,100 seems ok for the output buffer. But the input buffer needs to be much larger else it overflows. Also, an extra stop stream before terminating to make sure all audio from the audio buffer is played.
Merge pull request #38 from jpwichern/master

Portaudio support