Declarative bcon_append(...)

a varargs bcon_append with a ton of tasty magic.  bcon_append methods
offer declarative bson appending, including nesting, via tokenizing of a
varargs parameter stream.  Some limited macro support to make everything
a little less fragile.

Base functionality relies only on c89 stdarg.h.  Macro support is a
little more limited, but should still be available on any reasonably new
compiler (msvc 2005 and gcc > 3).  The base functions are usable without
their macro wrappers.