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.
See [above](https://github.com/SpongePowered/SpongeAPI/pull/304/files#r22119277)