Rearrange documentation

1) Moves all READMEs to docs/
2) Renames them to *.md, adds some Markdown with the idea to add a lot more
3) Moves the doxyfile config to doc/ and makes it parse the headers at ../include as well as the md files in docs.
4) Skips SDL_opengl*.h headers from the docs
5) Minor fixes to doxyfile
Moved documentation to docs, recreated a simple README.txt, fixed build
XAudio2: Use XAUDIO2_VOICE_NOSAMPLESPLAYED when possible.

For versions of XAudio2 with an IXAudio2SourceVoice::GetState() that offers a
flags parameter, we can use XAUDIO2_VOICE_NOSAMPLESPLAYED, since we don't
need this information in our current calls. According to MSDN, this makes the
the call about 3x faster.
Added a GetPendingBytes method to the audio backend.

This will (eventually) make SDL_GetQueuedAudioSize() more accurate, and thus
reduce latency. Right now this isn't implemented anywhere, so we assume data
fed to the audio callback is consumed by the hardware and immediately played
to completion.
SDL_GetQueuedAudioSize() shouldn't grab lock when not set up for queueing.
Fixed comment typo.
Make SDL_SysWMinfo usable on Mac/iOS with ARC enabled (thanks, Alex!).

Fixes Bugzilla #2641.
SDL - fix re-entrancy into SDL_UpdateFullscreenMode under OSX. During HideWindow we get a RESTORED event which then turns fullscreen back on causing a hang in Cocoa_SetWindowFullscreenSpace waiting for the fullscreen transition to finish.
SDL - when raising the window under OSX also force the app to activate. This fixes a fullscreen window on a separate space not coming front when raisewindow is called.
Haptics aren't available on iOS, but use the dummy implementation instead of failing init if it's requested.
The OpenGL context returned by the UIKit backend is now an actual OpenGL context instead of the OpenGL view we created.
This allows you to use the returned context in functions like CVOpenGLESTextureCacheCreate()
Added NV12 and NV21 texture support for OpenGL and OpenGL ES 2.0 renderers