Implement more lua API functions

[wateret]

• Retrieve Stage Information
  • GetUnitOnPosition(pos) : Get UId on the position
  • GetTerrainOnPosition(pos) : Get terrain info on the position
  • Extend UnitInfo object : Currently only contains id, uid, position
    • level
    • hero class
    • hero attribute (base)
    • unit attribute (base and current)
    • hp/mp (including current/max)
    • attack range
• Pushing Cmds
  • Kill a unit right away