Changed structure of Entity and Tile classes.
Removed the height and width parameters from Entity class. The preferred practice for now is to set up the sprite's source, size, offset, etc in the constructor of the child class and just pass in the finished sprite.

Also split up parts of updateOffset() into separate methods: one that wraps sprite.offset and takes into account the default tile size, and another that extracts the appropriate offset based the tile's name.
Small error fix.
Created file for common-use methods.
Added Player class.
Currently not finished, this will be a rather more complicated class than the previous ones. Right now it needs a its own draw method and a walk method, which will be a little more involved to write.
Started sanitising tile name parameter.