debug.c: made output of coap_show_pdu() more readable
debug.c: made output of coap_show_pdu() more readable

The new format[0] was inspired by the discussion[1] on the IETF CoRE
mailing list. Changes include:

* Print human-readable message type and method/response code
* Output token included in {...} even if token is empty
* Print human-readable option types
* Print human-readable option values where possible
* Print human-readable Content-Format and Block options
* Print hex string included in <<...>> for binary content,
  human-readable string included in '...' for text content
* Use '::' as payload delimiter if message contains a payload

[0] http://www.ietf.org/mail-archive/web/core/current/msg06069.html
[1] http://www.ietf.org/mail-archive/web/core/current/msg06062.html

gebart/libcoap,authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap