Refactorings and add some tests for remove() methods

Add constructor for the class Node:
    Node(prev, item, next)

Use the new Node constructor in add(), addLast()
Added tests for the remove methods for the case when there is only
one node in the list
Add field 'last'.
Add private method getNode(int index).
Rearrange some tests.
Merge branch 'master' of https://github.com/Shortrope/Doubly-Linked-List

Conflicts:
	src/DblLList.java