Add val function

Setting the value of an expanding textarea fails to trigger any of the usual re-size callbacks.  I added a `val` function to expanding textarea so that you can set it and have it resize itself after it was set.

Usage:

    $('textarea').expandingTextarea();
    $('textarea').expandingTextarea('val', 'A long\n\nmultiline\n\n\nstring');

Afterward you will notice that the textarea has expanded to show the full `val`.