Added the plot function as a chainable property.

This implements a long-standing enhancement of being able to create a plot like this:

```js
$("#placeholder").plot(data, options);
```

This form supports standard jQuery chaining, making it quite convenient when the placeholder is the result of a stream of jQuery calls, or when additional calls need to be made after the plot is created.

The disadvantage of this form is that it provides no way to return the plot object.  This isn't a big deal, though, since users can easily retrieve it via the 'plot' data key, like this:

```js
var plot = $("#placeholder").plot(data, options).data("plot");
```