Add $.extend utility method

#### $.extend( [deep ], target, object1 [, objectN ] )

Merge the contents of two or more objects together into the first object.. <br />
$.extend utility method (use the iterative way rather than the recursive one used in both jquery and zepto) to avoid potential exceeds maximum call stack size limit. <br />
> @deep {Boolean}, If true, the merge becomes recursive (aka. deep copy). <br />
> @target {Object}, The object to extend. It will receive the new properties.  <br />
> @object1, {Object}, An object containing additional properties to merge in.  <br />
> @objectN, {Object}, Additional objects containing properties to merge in.  <br />

```js
var target = { one: 'patridge' },
    source = { two: 'turtle doves' }

$.extend(target, source)
//=> { one: 'patridge',
//     two: 'turtle doves' }
```