DynamicSoundEffectInstance for OpenAL

We implemented DynamicSoundEffectInstance for OpenAL on Mac and iOS along with some integration tests.

We used a partial class to encapsulate the OpenAL specifics to keep the code more readable than using #if.

There is a race condition when rapidly playing and pausing that has been difficult to debug due to various issues with Xamarin Studio on Mac. Other than that issue, tests pass on Mac OS for playing, stopping, pausing, resuming, and looping clips.

TODO:

Test on PSM
Test on iOS
Integrate the existing DirectX pull request
Move more OpenAL calls into the OALSoundBuffer class

The TODO items might not all be required to integrate this pull request. I'd like to get input on what other work should be done on this patch.