Create zones with WorldEdit cuboid selections

The WorldEdit API is utilized to allow the use of WorldEdit selections
when creating or modifying a warzone. If the current player has a
WorldEdit selection when the /setzone command is used, this selection will
be used for the zone's corners.
Make sure the selection is a cuboid
Merge pull request #641 from cmastudios/weselection

Create zones with WorldEdit cuboid selections. Closes #411. Thanks @cmastudios!