initial death marker code

working death marker implementation... the only thing I'd look to change in the future is to make the number of total death markers a configurable amount rather than hard coding a limit of 3. 

In a different local branch I've changed Marker a bit to allow an icon rather than a color (currently all the entity heads are the only options) the equals() method on Marker and the getMarkersInGroup method in MarkerManager are cherry picked from there since they made things easier-- I don't know if you have an opinion one way or another when it comes to using Guava; you don't make use of it anywhere so my getMarkersInGroup method introduces a dependency in MarkerManager but since MW is a Forge mod Guava will be available and it's not an issue... If you have plans to un-join MW and Forge for some reason I figured I'd mention it.