Update v2.1.1

• Fixed NBTList.set() throwing UnsupportedOperationException (#10, thanks @PureGero)
• Throw NullPointerException instead of IllegalArgumentException when argument is null. Applies to:
  • NBTCompound:
    • get() (and all other getters for that class)
    • put()
    • putIfAbsent()
    • containsKey()
    • containsValue()
    • containsTag()