Allow an authenticated user to post reviews and ratings for extensions

[jlong]

Rating System:

• Use a five star system
• Users can rate an extension multiple times, but only the latest rating counts
• The average rating is rounded, so an average of 2.5 stars should be displayed as 3 stars, while an average of of 2.1 stars should be displayed as 2 stars

Reviews:

• Sorted with the most recent review first
• Only 8 reviews should be shown
• An more link expands (javascript) to show all of the reviews

Other notes:

• Users should only be able to create a review/rating when they are logged in, a small message about this should be displayed when they are not

Shown here:
http://github.com/radiant/radiant-mockups/blob/master/registry-extension.png