Added deferred object and event to manifest model so I can get access to...

Hi, 
I wanted to see what the group thought of this change before I ran the grunt script to minify.   I need to be able to access the manifest outside of dash.js for a feature in the Akamai support player.  I add this code a while back to our local repo but never pushed to github for review.   This seems like something other player developers may need as well. 

Basically just added a deferred object and event when the manifest is loaded.