adds a schema to support full format and adds the ability to subscribe to particular tags

i added a [schema.js](https://github.com/antimatter15/js-ebml/blob/master/schema.js) from @antimatter15 [js-ebml](https://github.com/antimatter15/js-ebml) to support the full format and changed the emit calls to emit the tagName. now one can subscribe to particular tags like Cluster. i edited the example.js and test/decoder.js to reflect that changes.