Added User Story for MediaObject (+ can LDP help?)

Hi,

Since we work from User Stories, I added one to capture requirements of
MediaObject:
https://www.w3.org/wiki/Socialwg/Social_API/More_user_stories#Liking_a_Photo_found_in_an_Album

1. Jessy browses throught Ralph's album of photos titled "Visiting Bay Area"
2. Jessy Likes one of the photos titled "4/20 in Golden Gate Park"
3. Ralph now can see Jessy's like when visiting this Photo page

I think it makes it clear that we need, in LDP terminology[1]
* Linked Data Platform RDF Source (LDP-RS) - which has *Like* affordance
and provides count of likes
* Linked Data Platform Non-RDF Source (LDP-NR) - multiple one's which
provide encoding of that photo in various sizes and file types

I also hope that numerous members of LDP WG, also participating here can
help us with understanding if it LDP-RS and LDP-NR could help us with
solving MediaObject issue.

Cheers!

[1] http://www.w3.org/TR/ldp/#terms