src/linkedlist.c: updated description for ll_clear(...).
Queue Implementation

Added an implementation for a queue that is built on top of the previously
built linkedlist.

File(s):
   modified:   Makefile
   new file:   include/queue.h
   new file:   src/queue.c
Linkedlist Semantic Update

Updated the semantics of the code in the linkedlist implemenation.
Documentation inconsistencies were also updated.

File(s):
   modified:   include/linkedlist.h
   modified:   src/linkedlist.c
include/queue.h: semantic update.