set Product Qty field to type number

Good UX, especially for iPads etc., to bring up the correct keyboard.
Change to subclassing NumericField

In the hopes that Silverstripe will change NumericField to have type="number".
(see issue https://github.com/silverstripe/silverstripe-framework/issues/3289)
Merge pull request #68 from RuthAdele/patch-2

set Product Qty field to type number