list: add LastIndexOf (#46)

Add LastIndexOf method too List and fix doc

LastIndexOf returns the index of the last occurrence of the value in this list.
if not found return -1

datastructure/list/list_test.go

@@ -36,6 +36,23 @@ func TestIndexOf(t *testing.T) {
 	assert.Equal(-1, i)
 }
 
+func TestLastIndexOf(t *testing.T) {
+	assert := internal.NewAssert(t, "TestIndexOf")
+
+	list := NewList([]int{1, 2, 3, 3, 3, 3, 4, 5, 6, 9})
+	i := list.LastIndexOf(3)
+	assert.Equal(5, i)
+
+	i = list.LastIndexOf(10)
+	assert.Equal(-1, i)
+
+	i = list.LastIndexOf(4)
+	assert.Equal(6, i)
+
+	i = list.LastIndexOf(1)
+	assert.Equal(0, i)
+}
+
 func TestContain(t *testing.T) {
 	assert := internal.NewAssert(t, "TestContain")