Added PlayerDeathEvent

Cause fixed. Example:
You can get the type using ```e.getCause().get().getParent().get().getCause()```, which will return something like: ```explosion.player```
You can get the entity which caused the death using ```((Entity)e.getCause().get().getCause())```

In Sponge there is no death event yet so if there is a good way to send the cause i'll change it later. (This is a bit cheaty solution atm, because I think the Cause class is not finished yet)