Add VoidParameter type
Reorder test suite a bit
Fix some bugs in triggering observer callbacks

StringParameter was not notifying observers at all, and callbacks were
being made when setting the value to itself. Both issues have been
fixed with failing tests.
Remove unused field
Adding parameter to set returns that parameter

This can be used to chain calls together or to save the added parameter to a
local cache variable. The bool return value was seldom checked in actual use
anyway. Fixes issue #8.