Option length calculation fixed according to coap-18

Fixed an incorrect use of the option length parameter to
calculate option delta values > 13. According to CoAP
draft 18 section 3.1, both len and delta can have values
greater than 13. A quick test for 13 < len < 269 was
performed using libcoap.
Dynamically build CoRE discovery response at setup
Merge pull request #1 from jdeblese/master

Option length and delta calculation when > 13