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

the problem is that the cloned request still shares some state with the previous one, e.g. the `_packet` property, and other stuff. I think you can have a `nextRequestForBlock` function somewhere that creates a new one and duplicates all the relevant fields.