Add a spec to support namespaces in attr binding.

Attribute namespaces can be defined by `xmlns:<i>prefix</i>="namespace-url"`
attributes, and allow all descendents of that element to define
`<i>prefix</i>:name="value"` attributes.
Implement support for namespaces in attr binding.

The DOM provides a way to lookup which namespace an attribute name belongs to
given the element the attribute will be bound to, Node.lookupNamespaceURI
(http://www.w3.org/TR/DOM-Level-3-Core/core.html#Node3-lookupNamespaceURI).

Given the namespace, it's simple to add/remove the desired attribute with the
correct namespace using the Node.setElementNS and Node.removeAttributeNS.