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

I am new to nodejs, your suggestion may provide value to my coding :).

Do you think that try-catch is really that bad? I dont known how slow it is, but those parseBlock2 and createBlock2 are synchronous functions, and just like json parser.

Catch error by return null is not good enough, when, for instance, parseBlock2 return null as there's no block2 option. 
How about return an Error object, and check for typeof?

I tried callback(null, value) as recommended error handling (http://www.joyent.com/developers/node/design/errors), but callback makes  the rest handling code urgly.

Things will become more easy if js supports multiple return, e.g.
val, err = function()