Take 2: Fix issue #561 (bogus "Invalid Param name" error)

First attempt was both broken and ill-conceived.
Add Parameter::operator=
Removed Parameter::unregister_instance() in favor of a actor argument.
Adding Parameter comments.
s/Param/Parameter/
Merge pull request #563 from halide/usercontext

Take 3: Fix issue #561 (bogus "Invalid Param name" error)