Don't create ItemStack NBT during place if original is null. Fixes #1292
Fix possible crash double-clicking in modded inventories. Fixes #1274

The Bukkit inventory event checks for InventoryAction.COLLECT_TO_CURSOR
by examining the contents of the inventory, using IInventory getContents().
However, this method is added by CraftBukkit and therefore is not available
on modded inventories. Catch the AbstractMethodError and set to
InventoryAction.NOTHING with ClickType.DOUBLE_CLICK instead.
Fixed Aether Login NPE. Fixes #1205
Improve error message from invalid MCPC+ installation

If the vanilla Forge jar is (erroneously) extracted into the MCPC+
jar, probably from an ill-fated attempt to update Forge, critical classes
will be overwritten and the server will fail to load. The failure
could occur with NoSuchMethodError(or field/class) anywhere, but our custom
MCPC+ code is first called in the FML relauncher, so if it is overwritten
we'll fail with: java.lang.NoSuchMethodError: cpw.mods.fml.relauncher.FMLInjectionData.obf151()Z

This commit detects this error and shows a message to the user asking
to retest with a clean MCPC+ jar, as a helpful hint in the right direction.
Check for null username when calling areCommandsAllowed. Fixes #1205
Add isAirBlock check in isEmpty via Bukkit API Block. Closes #1284