created "initalize" methods to make explicit what is happening in constructors.
fixed MediaProps's private instance reference.
added comment to document reason for last commit.
Made "this.config" into a private property now called '_config'
Broke "MediaProps" constructor out of Embederator constructor, and renamed it "AssetProps"

* renamed other constructors -- "media" refer to collection of multiple "assets"
'MediaConfig' is now a private method.   (b/c config occurs only *once*, when single instance of Embederator is constructed?)