* test/unit/Makefile.am, test/unit/expr/attribute_white.h,

* test/unit/Makefile.am, test/unit/expr/attribute_white.h,
  test/unit/expr/node_white.h: add whitebox attribute test (pulled out
  attribute stuff from node_white)

* test/unit/parser/parser_black.h: fix memory leaks uncovered by
  valgrind

* src/theory/interrupted.h: actually make this "lightweight" (not
  derived from CVC4::Exception), as promised in my last commit

* src/theory/uf/theory_uf.h, test/unit/expr/attribute_black.h: match
  the new-style cleanup function definition

* src/expr/attribute.cpp, src/expr/attribute.h: support for attribute
  deletion, custom cleanup functions, clearer cleanup function
  definition.

* src/expr/node_manager.h, src/expr/node_manager.cpp: reclaim
  remaining zombies in dtor, rename NodeValueSet ==> "NodeValuePool",
  and enable freeing of NodeValues

* src/expr/type.h, src/expr/type.cpp: reference-counting for types,
  customized cleanup function for types, also code cleanup

* (various): changed "const Type*" to "Type*" (to enable
  reference-counting etc.  Types are still immutable.)

* src/util/output.h: add ::isOn()-- which queries whether a
  Debug/Trace flag is currently on or not.

* src/smt/smt_engine.cpp, src/parser/antlr_parser.cpp,
  src/expr/type.cpp, src/expr/expr_manager.cpp, various others:
  minor code cleanup