Spawn DungeonLevelCharacterActors into DungeonLevelActor

* DungeonLevelActor assigns characters to a specific of the stage
Check if a Tile exists before spawning character
DungeonLevel assigns Tile to DungeonLevelCharacter

* DungeonLevelCharacter is no longer instantiated with x & y coordinates
* DungeonLevelCharacter Tile is assigned by DungeonLevel when spawning on it
Forbid movement on inexistant Tiles

* Dungeon::Level::Tile now is now aware of its DungeonLevelActor
Add some missing TomDoc, change DungeonLevelActor#tile? parameter names
Merge pull request #43 from rafaelgonzalez/restrict-character-movement-with-tiledmap

Restrict character movement within tiledmap