Sending NONCE and TTL in the payload for active presence:

Sending NONCE and TTL in the payload for active presence:

Patch set #1:
1) The payload is formatted as NONCE#:TTL#
2) NONCE is a 32bit unisgned random int.
3) The client app is not called when there is not change in NONCE, OC_STACK_PRESENCE_NO_UPDATE was deleted
4) Code organization in occoap.c
5) Rebased on master

Change-Id: I2c4f8c4b30c3d692a38b3286001d55f3387f5a73