Add support for step for any number field

For a number field that is representing a floating point number, the
step needs to be set to a value smaller than 1 or to "any",
otherwise the browser will reject any decimal places.
Merge pull request #19 from kynan/step-for-number-field

Number input elements support a step attribute, too, apparently.
Use . notation for ng-model to allow nested models
Handle assignments of model value using the val attribute