RealFort
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	RealFort was designed to deal with the lackluster benefits of building forts in vanilla Civ IV.  It
provides extra bonuses for units that are within the fortifications.

Fort Bonus I:
+10% strength
+10% healing rate
+1 visibility range

Fort Bonus II (requires Optics):
+10% strength
+1 visibility range

Fort Bonus III (requires Steel):
immune to first strike
+1 first strike
10% withdrawal chance

v0.2
- spread bonuses over three promotions instead of one
- fixed bug that allowed building fort, getting fort promotion, then switching to different tile improvement
  and keeping promotion
- fort promotions two and three are now tied to specific techs

v0.1
- added fort promotion bonus