Fix example in the looping scene spec
Update the documentation of Core::Map#visible_tiles
Make more clarifications in the documentation of Core::Map#visible_tiles
Fix Map#find_tileset so it doesn't exceed the size of the tileset
Add spec for Core::Map
Add parentheses around arguments when they span to the next line
Add paretheses around arguments that span on multiple lines
Use before only for the examples that need it
Avoid using before whenever possible