make Validator#validator() chainable
allow Field to be context for val fn generator
allowing 'required' fn to take non-strings (like checkbox bools)
adding/removing the 'novalidate' attribute (to prevent browser validation from clashing)
Merge pull request #10 from dominicbarnes/small_enhancements

Some small enhancements
0.9.0

* change `#validator` to be chainable
* change `required` validator to handle non-strings
* remove `novalidate` attribute from forms
* call validators with `Field` as context