Board limits for staff, owner-only pick-up and update checks

• /skateboard here puts a board down without an item. It needs skateboards.here (ops by default), and skateboards.here.unlimited skips the limits below.
• Boards made by command have a cooldown and a per-player cap, set under spawn-limits in the config (10 seconds and 2 boards by default).
• Putting a board down now asks protection plugins first, the same check RP Engine makes for its own items, so WorldGuard regions and lobby guards keep boards out.
• Only the player who put a board down can pick it up. skateboards.pickup.others lets staff pick up anyone's.
• Ops are told when a newer version is on SpigotMC (skateboards.update). Set update-check: false to turn it off.
• Needs RP Engine 0.2.3 or newer.