User story: As API user, I want to get unique ID of the content relative to a site.
Examples
https://www.youtube.com/watch?v=XOmwZopzcTA&index=9&list=PL53194065BA276ACA
ID: XOmwZopzcTA
https://soundcloud.com/jackedradio/afrojack-presents-jacked-radio-week-23
ID: 210140747
For a site where it's not possible to recognize system ID it would be based on a value from URL:
https://soundcloud.com/jackedradio/afrojack-presents-jacked-radio-week-23
ID: jackedradio/afrojack-presents-jacked-radio-week-23
What do you think?