XP simplification

Don't treat experienced units as more valuable for the purpose of experience
Each rank is making cost once
Morph: XP carries over sensibly

Scaling is now based on relative cost (ie. the new unit has exactly as much XP as if would have if it killed everything the old one did).
Amount of XP carried over is not capped (this means chain morphs are allowed, albeit hard due to previous).
Merge pull request #228 from sprunk/patch-7

Simplify XP and morph