Make StereoPannerNode a-rate and spec the algorithm

It was somewhat obvious, but it's clearer to also have the formulas for [-1, +1]

This fixes #439.
Fix typos
Fix typos added when fixing typos
Fix algorithm issue for stereo->stereo
Merge pull request #440 from WebAudio/439-stereoPannerNode

Make StereoPannerNode a-rate and spec the algorithm