Fix game not using UnitFactory for creating unis

In playCard method.
Add "powers" mechanics

- Units now may have special "powers", triggered on some events. Powers
  have the same effect as Spells do. Just topmost spell revives "this"
  as target before "exequte" method is called.
- Triggering events so far:
  * OnCreate (unit spawns), OnAllySpawn
  * OnTurnStart, OnTurnEnd
  * OnDamage, OnAllyDamage
Enabled powers reading from XML file

Unit's description now may have "Power" tag with "condition"
containing name of event, when this power should be triggered.