block2 request

coap.js now can request for a resource that return in blockwise(2).

Tested with coap.request('coap://coap.me/.well-known/core').

Todo
- block2 for observe
- block1

When res.end(message) invoked, and this message got fragmented with blockwise, this kind of fragment should be dealt by server.js, IMO. A cache is needed. The key is somewhat related to ETag.
The later blockwise request for a cached payload should be returned directly, not routed to upper layer.