Allow easier turn on of #define NS* declarations

[robertjpayne]

Currently you must modified the DDXML.h header to turn on the #define NS* declarations.

Instead of:

#if TARGET_OS_IPHONE && 0 // Disabled by default

Do:

#ifndef DDXML_NS_DECLARATIONS_ENABLED
#define DDXML_NS_DECLARATIONS_ENABLED 0 // Disabled by default
#endif

#if DDXML_NS_DECLARATIONS_ENABLED

Which would allow "turn on" without modifying the headers. This is important so KissXML can be easily updated and not "patched" over and over to turn it on.

[anagromataf]

Would be nice to have this modification.