Diplomacy

I suggest we change the condition for conducting diplomacy from having an
embassy (as now) to merely having made contact.

This will be a big change of gameplay; but that is also the only good
reason I can see against it.

Reasons for:
  - It was this way in Civ2 (duh).
  - Diplomats less important.
  - No longer have to micromanage a diplomat into the land of each enemy
to do diplomacy with him.
  - It will be very hard to get diplomats to establish embassies with AIs
(and players) with autoattack.
  - AI diplomacy (coming soon) is much more fun this way.

If nobody objects, I'll put this change into the AI diplomacy patch (two
lines of code change).

  - Per