[development] subscribe to a node

[Moshe Weitzman]

> i'd *really* like to avoid implementing these from scratch in 
> project_issue.module and project_release.module, respectively.

well, you wouldn't do it there. you would do it so all node types could 
benefit but i get your point.

> 
> dries, killes, et al, can we open a discussion about running the 
> subscriptions.module (or equivalent, if there's a better alternative in 
> contrib already) on d.o, so i don't have to re-invent the wheel, just 
> for the sake of being "inside" project*.module?

that discussion is easy - subscriptions module has some ugly code in it and 
  needs significant cleanup. Ideally, the best parts are salvaged and 
submitted to core as a patch. i wouldn't run it on groups site, and i'm more 
aggressive than d.o. volunteers are encouraged to work up this patch.

-moshe