#1 Heroes and new XML possibilities

[RainCT (Siegfried Gevatter)]

Make it possible to have heroes. Here some ideas:

- They can be in the /units/ folder or in a new /heroes/ folder. They can be defined in the tech.xml & magic.xml.

- Each limited by: Only X at start / Only X can be trained in the game / Only X at current moment, if one dead, you can get another. This makes it possible to have single heroes but also to build special units (for ex., a group of elite chevaliers).

- Think it should be easy to use them in campaigns.