gen-art-13+14+15+17+18

- "a node constructs" -> "a node MUST construct".
- "resulting message will use the normal overlay routing" ->
  "resulting message MUST use the normal overlay routing".
- "The node can also" -> "The node MAY also".
- "Because messages> may" -> "Because messages can".
- "the request is retransmitted" -> "the request MUST be
  retransmitted".

Closes #149
Closes #150
Closes #151
Closes #153
Closes #154
Merge pull request #69 from deanwillis/gen-art-13+14+15+17+18

gen-art-13+14+15+17+18