Functions for removing and updating hashtable item

Added functions ck_remove_item() and ck_update_item().
Refactored code of find functions:
  - private functions now return non-const ck_hash_table_item **
    (used in ck_update_item() and ck_remove_item())
  - ck_find_item() only checks the pointer and returns const
    pointer

Added test for removal (test_remove()) and changed the main
  testing function to test_fnc_from_file() which is given pointer
  to the function to use for testing (used for insert and remove).
Added some comments to hash table.