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

Yes, I just clone _packet, which duplicates request fields, e.g. path, query. The only differences are token (omitted) and message id (increased); therefore, cloning _packet (not = operation) will save code space (but require additional underscore module)

Having nextRequestForBlock function is good. I will implement this.