Fix: Return hash of element after insertion
Added interface definition for ht_ndel()
Merge pull request #117 from matthiasbeyer/add-ht_ndel

Added interface definition for ht_ndel()
Fix: The hash function gets void* and returns rs_hash
The hashtable does not need to know the datasize
We need a compare element and the cfg as well when finding elements
We return the found element or NULL
Fix: ht_del() gets hash, cmp element, cfg and returns the element which was deleted
Fix: ht_insert() needs the cfg as well
Fix: We should include the set header here as well
Removed ht_find() implementation - reimplementation required
Fix: Return number of removed elements on ht_del()
Return status instead of hash

Due to the discussion in #107, we should return a status code here
instead of the hash.
Added missing include
Merge pull request #107 from matthiasbeyer/fix-ht_interface

[WIP] Fix ht interface
Added alias for nonnull argument attribute macro
Fixed typo
Fixed comments of avl_del function interface
Updated function interface description of avl_find
Removed ternary operator due in avl_height()