code: function World( cfg, fn ){
  
          // allow creation of world without "new"
          if (!(this instanceof World)){
              return new World( cfg, fn );
          }
  
          this.init( cfg, fn );
      }
docs: begin world definitions alias of: Physics
  new Physics.world([options, fn(world, Physics)])
  - options (Object): configuration options (see description)
  - fn (Function|Array): Callback function or array of callbacks called with this === world
  - world (Physics.world): The current world created
  - Physics (Physics): The Physics namespace
  
  World Constructor.
  
  Use [[Physics]] to create worlds.
  
  Configuration options and defaults:
  
  ```javascript
  {
  // default timestep
  timestep: 6,
  // maximum number of iterations per step
  maxIPF: 4,
  
  // default integrator
  integrator: 'verlet',
  
  // is sleeping disabled?
  sleepDisabled: false,
  // speed at which bodies wake up
  sleepSpeedLimit: 0.1,
  // variance in position below which bodies fall asleep
  sleepVarianceLimit: 2,
  // time (ms) before sleepy bodies fall asleep
  sleepTimeLimit: 500
  }
  ```
  
  If called with an array of functions, and any functions
  return a [promise-like object](http://promises-aplus.github.io/promises-spec/),
  each remaining callback will be called only when that promise is resolved.
  
  Example:
  
  ```javascript
  // hypothetical resources need to be loaded...
  Physics( cfg, [
  function( world ){
  var dfd = $.Deferred()
  ,images = []
  ,toLoad = myImages.length
  ,callback = function(){
  toLoad--;
  // wait for all images to be loaded
  if ( toLoad <= 0 ){
  dfd.resolve();
  }
  }
  ;
  
  // load images
  $.each(myImages, function( src ){
  var img = new Image();
  img.onload = callback;
  img.src = src;
  });
  
  return dfd.promise();
  },
  function( world ){
  // won't be executed until images are loaded
  // initialize world... etc...
  }
  ]);
  ```