Tilemap.loadString - Ingore negative values

Tilemap.loadString ln 303
"Ogmo Editor" outputs -1 in it's string if a tile is not present, the tilemap class handles this poorly due to the uint conversion. Proposed change handles negative values by ignoring them and not adding a tile.
Tilemap.as change loadFromString to handle negatives

Advice given by zachwlewis
Merge pull request #120 from SmileyJames/patch-1

Tilemap.loadString - Ingore negative values