Added alias for nonnull argument attribute macro
Revert order of includes so we do not override stdlib macros here
Include attributes header
Merge pull request #127 from matthiasbeyer/add-nonnull_attribute

Added alias for nonnull argument attribute macro
Add __nonnull__ attributes to functions of ll.h
Merge pull request #141 from neithernut/add-attributes-to-LL

Add __nonnull__ attributes to functions of ll.h
Add ll_is_empty
Use ll_is_empty in avl_del
Merge pull request #146 from neithernut/add-ll-is_empty

Add ll_is_empty
Added interface definition for ll_ndel()
Added implementation for ll_ndel()
Added test for testing deletion by predicate
Added implementation of ht_del()
Added debug print helper for avl.c
Added debug printing for avl_find()
Added debug printing for avl_add()
Added debug printing for avl_del()
Added debug printing for insert_element_into_tree()
Added debug printing for destroy_subtree()
Added debug printing for rebalance_subtree()