Add support for binding input value as numbers

In the bind-input example, bound layer properties should be numbers, but when inputs of type range are modified, layer properties will be set with strings instead of numbers. This pull request fix this.

This makes use of the valueAsNumber HTMLInputElement property introduced in HTML5 spec:
http://www.w3.org/TR/2011/WD-html5-20110405/common-input-element-attributes.html#dom-input-valueasnumber

Please review.