fix DrawGrid

the grid was extending past the edge
for example Demo7: http://i.imgur.com/QdoOlzY.png
this also changes color & draws an edge
http://i.imgur.com/iHIkpCl.png

i guess maybe i make assumption that tileSize = unitSize =  PixelsPerMeter which it does but

this has nice selection and grid
http://wiki.unity3d.com/index.php/2D_Tilemap_Starter_Kit