Use API v3 and include required user agent header.

Envato have released v3 of their API which breaks this wrapper because v3 now requires a user agent header to be sent to use either the edge or v3 release of their API. Since edge is subject to constant change, it could break this wrapper again without warning, because of this I think it's best to specify a stable release, hence why I have also changed the wrapper to use the v3 release set.