Updated user's guide.

Updated Niflib to the latest XML which adds support for bhkMeshShape, NiMultiTextureProperty, NiRawImageData, and NiSequence.

Changed the default linkage to DLL.  You now have to define NIFLIB_STATIC_LINK if you want to use static linking instead of having to define USE_NIFLIB_DLL if you want to use the DLL.
Added a new Debug - DLL configuration.
Static configurations now create either niflib_static.lib or niflib_static_debug.lib, while DLL configurations create niflib_dll.lib and niflib_dll_debug.lib along with niflib.dll and niflib_debug.dll.

* fixed detection of visibility=hidden support (gcc, linux)
* compilation will give error message if visibility=hidden is not detected