implement prop: and attr: namespace for props

[brainkim]

Implements #281

[brainkim]

Thought a little bit about whether it’s wise to use an attribute namespace (prop:, attr:). It could potentially cause issues with exotic XML things, but I think it’s better than the alternatives. prop$, attr$ is ugly and not as clear. . and # are invalid characters. prop- and attr- could potentially have similar issues. prop:/attr: is probably fine and only requires that JSX parsers don’t throwIfNamespace. Crank.js already doesn’t have a problem with namespaced attributes so this is a non-issue.