Included /observedir as a resource to list or reset observation relationships
Usage: Get requests to /observedir using queries action=list and action=reset may be used to get the desired output.
	Eg:	coap://<serverIP>:[<port>]/observedir?action=list - lists all existing relationships
		coap://<serverIP>:[<port>]/observedir?action=clear - resets all existing relationships
Merge branch 'coap13' of https://github.com/fvdnabee/contiki-sensinode into coap13

Conflicts:
	apps/er-coap-13/er-coap-13-engine.c
	apps/er-coap-13/er-coap-13-observing.c
	apps/er-coap-13/er-coap-13-observing.h
	apps/erbium/erbium.h
Added Client and Server examples
Server modified by Jen