p2 added

p2: Find the last but one element of a list.
p3 added

p3: the K'th element of a list. The first element in the list is number
1.
change parameters

eg: (x:xs) to (_:xs)
finish p4

p4: Find the number of elements of a list.
finish p5

p5: Reverse a list.
finish p6

p6: Find out whether a list is a palindrome.