Activity and Verb Metadata

For https://github.com/adlnet/xAPI-Spec/issues/35

* changed metadata to JSON from XML within spec
* defined JSON format
* allowed any type of metadata at URI (LRMI, microdata, RDF, etc) (for activities, not listed, but implied)
* did the same for verb, etc... metadata -- except talking about "any format", do we want that here?
* ADD ‘url’ field to activity definition for ‘read more’ or ‘launch

Still todo, should leave the issue open to track these even if we merge:
TODO: 5 - create 2 content types (application/json+XXX)
TODO: 6 a- it may simply be a human readable document