[sugar] API policy

[Marco Pesenti Gritti]

I updated according to the discussion in the irc meeting and put it on the wiki:

http://sugarlabs.org/go/DevelopmentTeam/API_policy

Also I marked the sugar package modules as described above.

Please make sure to have a look, we will need to maintain the
stability promises we are making. In particular, I marked the
datastore dbus service as stable, but I'm not sure we reached
consensus on that one in the meeting. Let's discuss it if anyone
disagree with that choice.

Marco