minor update to SamplePlayer docs
added VoiceTest
minor comment update
updated pch files for NodeTest, since they get copied to new tests
Voice is stored by weak_ptr in MixerImpl, removed when it goes out of scope
removed MixerImpl's master gain, it wasn't exposed and nothing like this will be in public api for initial release
added ability for Voice to take option that tells it not to connect to master output.  Voice now exposes getInputNode() and getOutputNode(), user can connect up the Voice themselves with the latter.
added a stereo source file to the test
added VoiceTest to msw test sln