Add ability to conditionally validate certain form fields

This potential feature adds the ability to conditionally skip validations of certain form fields.

I am building a complicated form in which certain fields may or may not be present on the page based on the user's previous behavior and/or other logic from the server. What I wanted was an easy way to say "form field X should be required, and alphanumeric, and have a max length of 8, ONLY IF condition Y is true"

I found this approach provided me with a higher degree of control than registering callbacks for my particular use case.

Here's the syntax I came up with:

```
{
  name: 'name',
  rules: 'required',
  depends: function() {
    /*  this form field will be run 
        against the validation rules
        only if the function returns true
    */
  }
}
```

You can also pass a string into depends, ie: 'depends: myfunction' and then use form.registerConditional to register a function which will be called in the same manner.

This is my first OSS commit. Please let me know your thoughts!