Add an option group to the status key to allow separation of prefetch status from normal page status.

Closes #116 
@nicksay I felt that the function had an awkward name/use case combination. The name implied some generic prefixing capability, but really only built keys from one specific combination. In fact, many of the users refer to the output as 'key'.

So instead I figured it would make more sense as a generic key maker for resources. Which would work equally well for any set of labels that were being sent to it. I wouldn't necessarily be to opposed to

spf.net.resource.key = function(type, label, opt_group)

But, I would put money on a new variable being needed soon enough, and we'd revisit it again.