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

try-catch makes the function not-optimizable by V8, it's not bad on its own, it's just bad on performance.

maybe we can just move the logic for iterating across the options from parseBlock2 to the outer method. So that parseBlock2 has only two possible outcome, the parsed value or null in case of error.