Use number constraint for HTML5 number inputs if an appropriate 'step' attribute is set

This change adds a condition that uses the "number" constraint for HTML5 number inputs if the `step` attribute is set to "any" or a non-integer value.

Per the [specification](http://www.w3.org/TR/html5/forms.html#the-step-attribute), floating-point values are allowed in HTML5 number inputs if the [`step` attribute](https://developer.mozilla.org/en/docs/Web/HTML/Element/Input#attr-step) is set to "any" or a non-integer value. 

Parsley incorrectly defaults to using the "integer" constraint for HTML5 number inputs, and this behaviour can't be overridden by setting a `data-parsley-type` attribute on the field.