Added ItemConverter tests.

- Changed Slot.Weapon to Slot.Hands (more descriptive, particularly for
  Shields).
- Moved TestSerialization and TestDeserialization to TestConverter and
  made TestSerializationSource and TestDeserializationSource abstract
  methods. Made corresponding changes in subclasses converters.
- TestSerialization (in Integration rather than Unit now) now loads
  assembly via type rather than name (not sure why).
- Added TestItemConverter.
- Changed Character.ToString() to put details on one line for easier
  display.