[SDL] Joystick API implementation using HTML5 Gamepad API

Works in browsers that implement the working draft of the standard (current Chrome / Firefox stable):
http://www.w3.org/TR/2012/WD-gamepad-20120529/#gamepad-interface

...and browsers that implement the editor's draft (current Firefox Nightly):
https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html#idl-def-Gamepad

Contains unit tests for both event types.
Merge branch 'sdl_joystick' of github.com:jvilk/emscripten into incoming
version 1.7.3
fix joystick browser integration