add more valid values for the "render" property

[mamund]

Cj defines valid values for the render property as "image" and "link". The following should be considered to add to the list of valid values:

"none" : this link is not for visual display (e.g. atom feed links, stylesheets, etc.)
"video" : render the content within a video player
"audio" : render the content within an audio player
"frame" : render the content within an embedded frame

There may be other values to consider, I'm open to discuss.

The current spec is silent on the topic of what to do w/ unknown values (bummer). It should be changed from:

"If the render property does not appear on a links array element, it should be assumed to be set to link."

to

If the render property does not appear on a link element or the value of the render property is unknown, it should be assumed to be set to "link".

This may have a negative impact on existing Cj clients(e.g. new values will appear and clients may not know how to handle them) and should be discussed.