Added errorMessageTemplate option for validate() method - allow custom HTML for generated errors.

Allows to create custom HTML structures as error messages.
Example:

$("#my_form").validate({
    rules: { username: 'required'  },
    messages: { username: { required: "Username is required"  }  },
    errorMessageTemplate: "&lt;a href='#'&gt; {0} &lt;/a&gt;"
} )

The corresponding generated error HTML:

&lt;label for=&quot;username&quot; generated=&quot;true&quot; class=&quot;error&quot;&gt;&lt;a href=&quot;#&quot;&gt; Username is required &lt;/a&gt;&lt;/label&gt;