This was not working because floating-point arguments are really passed in the extended format, but based on the wording in the C standard a type of "double" should still work for arguments passed with that type.
This fixes#29. (The bug report was valid only with respect to double, not float or long double.)
*Use a typedef rather than a macro definition for va_list. (The C standards specify that va_list is a type, although this would make a practical difference only if someone #undef'd it.)
*Don't include a semicolon in va_start(), so it expands to an expression rather than a statement. This could make a difference in a construct like "if (...) va_start(...); else ...".
