Initial merge of Emscripten port!

With this commit, you can compile SDL2 with Emscripten
( http://emscripten.org/ ), and make your SDL-based C/C++ program
into a web app.

This port was due to the efforts of several people, including: Charlie Birks,
Sathyanarayanan Gunasekaran, Jukka Jyl?nki, Alon Zakai, Edward Rudd,
Bruce Mitchener, and Martin Gerhardy. (Thanks, everyone!)

test/emscripten/joystick-pre.js

@@ -0,0 +1,25 @@
+Module['arguments'] = ['0'];
+//Gamepads don't appear until a button is pressed and the joystick/gamepad tests expect one to be connected
+Module['preRun'].push(function()
+{
+    Module['print']("Waiting for gamepad...");
+    Module['addRunDependency']("gamepad");
+    window.addEventListener('gamepadconnected', function()
+    {
+        //OK, got one
+        Module['removeRunDependency']("gamepad");
+    }, false);
+
+    //chrome
+    if(!!navigator.webkitGetGamepads)
+    {
+        var timeout = function()
+        {
+            if(navigator.webkitGetGamepads()[0] !== undefined)
+                Module['removeRunDependency']("gamepad");
+            else
+                setTimeout(timeout, 100);
+        }
+        setTimeout(timeout, 100);
+    }
+});