reference FSP-1037 draft 3 as the implemented standard

Rationale:

* a random visitor should have a rough idea of what standard is implemented by the module;

* it should be possible to reference the exact parts of the standard (by a line number) in comments. (For example, in https://github.com/askovpen/node-fidonet-squish/pull/1#issuecomment-45431235 I could refer to `Squish.txt` line number 453 where the `replies` array is mentioned.)