Stop putting properties on WebGLObjects

The problem with putting properties on WebGLObjects is your app
will crash if you ever decide to handle WebGLContextLost events.
The reason is all of the gl.createXXX functions will return null
and so code like this

     var buffer = gl.createBuffer();
     buffer.itemSize = 3;

Will crash because buffer will be null. The simplest solution
is to use JavaScript objects. So for example instead of

     var buffer = gl.createBuffer();
     buffer.itemSize = 3;
     gl.bindBuffer(..., buffer);

We can change it to

     var buffer = { id: gl.createBuffer(); };
     buffer.itemSize = 3;
     gl.bindBuffer(..., buffer.id);

That at least is future proof code.
Merge pull request #4 from greggman/no-gl-properties

Stop putting properties on WebGLObjects