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

programmatically try-catching and throwing exceptions is really a bad idea in node.js, it slows everything down.
So, can you please refactor the code to not throw? You might return null from parseBlock2, and then handle that case.