add thread_shutdown() to python

see https://github.com/jcupitt/libvips/issues/170

ChangeLog

@@ -6,6 +6,7 @@
 - fix memleak in webp load from buffer [Lovell]
 - fix memleak in png save to buffer [Lovell]
 - make out of order read in png and jpg a fatal error
+- add thread_shutdown() to C++ (and thereby to python)
 
 12/8/14 started 7.40.6
 - more doc fixes

libvipsCC/VImage.cc

@@ -64,6 +64,11 @@ void shutdown()
 	vips_shutdown(); 
 }
 
+void thread_shutdown()
+{
+	vips_thread_shutdown(); 
+}
+
 void VImage::refblock::debug_print()
 {
 	std::list<refblock *>::iterator i;

libvipsCC/include/vips/VImage.h

@@ -63,6 +63,7 @@ VIPS_NAMESPACE_START
 /* vips_init() and vips_shutdown as namespaced C++ functions.
  */
 bool init( const char *argv0 = "nothing" );
+void thread_shutdown( void ); 
 void shutdown( void ); 
 
 /* A VIPS callback, our name for im_callback_fn.
@@ -451,6 +452,8 @@ VIPS_NAMESPACE_END
 // Other VIPS protos we need 
 extern "C" {
 extern int im_init_world( const char *argv0 ); 
+extern int im_init_world( const char *argv0 ); 
+extern void vips_thread_shutdown( void ); 
 extern void im__print_all(); 
 extern void im_col_Lab2XYZ( 
 	float, float, float,