introduce auto print

Hi, 
I needed an auto print feature for my PDF. I tried many things :
* including iframe tricks [printing an overview of the webpage]
* jspdf.plugin.javascript with a call to this.print() [compatible acrobat reader only]

So you'll find a basic plugin. It needs some refinements (test/doc) but I wanted to check if you are interested. 

The basic idea for the auto printing is  
```
var doc = new jsPDF();
doc.text(20, 20, 'Hello world.');
doc.autoPrint()
doc.output("dataurlnewwindow") // so it open a new popup,  after this the PDF open the print window view. You can save the pdf and when opened, it display the print settings view.
```

Let me know what you need to merge.