"required" rule support for radio inputs, and "use strict" compliance

**A.** Make the code strict mode-compliant: declare variables before using them in "if" condition

I'm using validate.js as an AMD module (RequireJS), and wrap the code into a "use strict" function. Then, this:

    if (foo = bar()) {
        ...
    }

without explicitly declaring `var foo`, results in an error.

Also, some trailing whitespaces removed by my editor.

**B.** "required" rule support for radio inputs

Multiple radio inputs share the same name in the form, but vary in values; therefore `form["foo"]` is not an `HTMLInputElement`, but rather an array of `HTMLInputElement`-s.