Changes:

* Added a new interface "Convertible", with a single method "<T> T convertTo(Class<T> clazz)"
    * AttributeList.offer() now uses this to attempt to convert the given object into the desired one.
    * This is primarily used to fix the bug below.
* Deprecated FixedItemInvView.offerSelfAsAttribute and FixedFluidInvView.offerSelfAsAttribute because
  they now implement the convertible interface. 

Bug Fixes:

* Fixed SimpleFixedItemInvView.offerSelfAsAttribute adding two instances of ItemTransferable
  rather than just one.
