Add Web Audio API support

This pull adds support for the Web Audio API (http://www.w3.org/TR/webaudio/) into the System.Html.Media.Audio namespace.

Some key things to note:

* There's some browser detection in the [AudioContext](https://github.com/drysart/SaltarelleWeb/blob/879a260f9905b692d73b0a56b42c2f0edf2cd2b2/Web/Html/Media/Audio/AudioContext.cs) constructor.  Can't be avoided as the API is currently vendor-prefixed.

* There's a small correctness compromise with regards to the existing System.Html.AudioElement class not having a MediaElement parent class (as opposed to Element, as it currently has).  In the Web Audio specs there are a couple places where an HTMLMediaElement should be passed as an argument, and due to there being no HTMLMediaElement counterpart in System.Html, I simply defined the methods to accept System.Html.Element.   If/when a System.Html.VideoElement class (which also should have a MediaElement parent class) is added, this should be revisited.

* The TypedArrays previously defined under System.Html.Media.Graphics.WebGL were all buffed up to fully cover their specification.   There is one breaking change against the old version: [Int8Array](https://github.com/drysart/SaltarelleWeb/blob/842ed78eedeb63f5c02cdf88979dd0094cac19e6/Web/Html/Media/Graphics/WebGL/Int8Array.cs) was previously declared as having a constructor that takes byte[], but per the specs the constructor takes an sbyte[].   (The previously-missing [Uint8Array](https://github.com/drysart/SaltarelleWeb/blob/842ed78eedeb63f5c02cdf88979dd0094cac19e6/Web/Html/Media/Graphics/WebGL/Uint8Array.cs) class that correctly uses byte[] was added.)

* The definition of the TypedArray classes involves a lot of duplication across the various types (Int8, Uint8, Int16, Uint16, Int32, Uint32, Float32, and Float64).   I'm not sure what your policy is on T4 macros in the project, but their code could be greatly consolidated with one.

* Some missing functionality on [XmlHttpRequest](https://github.com/drysart/SaltarelleWeb/commit/8d5ace331a2ed8fef8d81e7eff38d48fe30b9df5#Web/Net/XmlHttpRequest.cs) was added -- particularly the .ReponseType and .Response properties to allow XHR to retrieve ArrayBuffers and Blobs from a remote server.

* These changes don't address it, but there's a pretty strong argument to be made that the TypedArrays stuff doesn't belong in the System.Html.Media.Graphics.WebGL namespace, and should go into a namespace where it's not mixed in with the WebGL stuff since it exists as its own standards specification, like say System.Collections.TypedArrays for instance.