Store the context of a proxy as an attribute on the proxy object.

This PR stores the `context` of a proxied function as an attribute on the resultant proxy.  The following shows why this is convinient:

```javascript
a = $.proxy(myFunction, myContext)
```

The context cannot be determined from `a` in *master*.  With this PR the context can now be determined by

```javascript
a.context
```

[Ticket #14552](http://bugs.jquery.com/ticket/14552)