WorldBorder and Potions

Added new method to set the startingDiameter, endDiameter and the time it will take [void setDiameter(double startDiameter, double endDiameter, long time);]
Added Splash potion option.
Changed Radius to Diameter.
Changed get/setBlockBuffer to get/setBuffer and changed the return and parameter to double as it is not measured in whole blocks.
Why have two diameters? There's only one diameter to resize to, implying the starting diameter would require setting the world border to the start diameter in 0 time, then resizing to the end diameter in x time... We shouldn't overload said functionality just for the sake of shortening something that has a small use case.