null check in CoAPClient.Discover()
move CoapObserveRelation to a separate file
fix wrong link format serialization

A comma will be incorrectly left in the end if the last resource is
filtered out.
sort resources and attributes in link format
modify unknown media representation in MediaType
prefix state propeties in Message with "Is"

The names without "Is" is planned to be used as related events, and the
names of those events by now to be names of corresponding methods.
update options in RFC 7252

Note that the Accept has been changed to 17 since draft 18.
add Size1/Size2 in Message