libpq++.h contained copies of the class declarations in the other libpq++
include files, which was bogus enough, but the declarations were not
completely in step with the real declarations.  Remove these in favor
of including the headers with #include.  Make PgConnection destructor
virtual (not absolutely necessary, but seems like a real good idea
considering the number of subclasses derived from it).  Give all classes
declared private copy constructors and assignment operators, to prevent
compiler from thinking it can copy these objects safely.