ensure an SDNV is actually a Buffer

* the SDNV works better if exposed as a Node.js Buffer
* change SDNV constructor function
* change failing tests to comply with the new API
* remove useless tests
update version on package.json
Merge branch 'master' of github.com:ruiquelhas/sdnv

* update to version 0.2 (using SDNVs as Buffers)