* src/expr/node_builder.h: fixed some overly-aggressive refcount decrementing.

* src/expr/node_builder.h: fixed some overly-aggressive refcount decrementing.
  There remain memory leaks (and some over-decrementing of refcounts) that
  I've identified; another commit forthcoming.

* src/expr/attribute.h: keys are now NodeValue* instead of TNode

* src/theory/output_channel.h: change OutputChannel::conflict() to the
  negation of what we had before: it now takes an AND of TRUE literals
  as a conflict clause rather than an OR of FALSE ones.

* src/expr/node.cpp: (non-template) CVC4::expr::debugPrint() routine
  for use inside gdb

* src/expr/node.h: remove Node::debugPrint() member (now a function
  instead of a member function, since Node is now a template it wasn't
  being properly instantiated(?) and couldn't be called from gdb)

* src/expr/Makefile.am: add node.cpp

* src/expr/node_manager.h: code formatting