add bsearch for arrays

Arrays can already be sorted with json_object_array_sort() which uses
qsort() of the standard C library. This adds a counterpart using the
bsearch() from C.

arraylist.h

@@ -49,6 +49,11 @@ array_list_length(struct array_list *al);
 extern void
 array_list_sort(struct array_list *arr, int(*compar)(const void *, const void *));
 
+extern void* array_list_bsearch( const void **key,
+		struct array_list *arr,
+		int (*sort_fn)(const void *, const void *) );
+
+
 #ifdef __cplusplus
 }
 #endif