Made player drowning implementation

In line with FS#142 the player now drowns when their oxygen runs out underwater. I'm using the block above the player to detect whether they are under water or not and have implemented drowning in line with the wiki listings [here](http://www.minecraftwiki.net/wiki/Drowning#Drowning) and [here](http://www.minecraftwiki.net/wiki/Chunk_format#Entity_Format).