Implement emscripten_async_wget2_data

This adds the new function emscripten_async_wget2_data to read from
an XMLHTTPRequest directly into memory while supporting advanced features.
fix C syntax in html5.h
Merge branch 'incoming' of https://github.com/kripken/emscripten into incoming

Conflicts:
	system/include/emscripten/emscripten.h