Allow async ajax calls and form class to be specified as string (URL argument).

I've written a patch that adds a semi-colon to the start of the JS so it can be included with other jquery plugins which might omit the closing semi-colon, adds `async` param to the validate method so that events like keyup, blur and change can trigger validation in the background while input continues, adds `method` param to the validate method so ajax validation can be used with GET method forms and views, allows `form_class` to be specified as a string so you can pass it as a URL argument or hard coded path rather than having to import the form class into urls.py. I also cleaned up white space in a few places.

The patch is available at: http://paste.pocoo.org/show/153778/