port GildedRose Java/Ruby to R language, add a RUnit test stub

R/item.R

@@ -0,0 +1,13 @@
+item <- function(name, sell_in, quality) {
+  newItem <- list(name=name, sell_in=sell_in, quality=quality)
+  class(newItem) <- 'item'
+  newItem
+}
+
+as.character.item <- function(item) {
+  paste(item$name, ", ", item$sell_in, ", ", item$quality, sep='')
+}
+
+print.item <- function(item) {
+  print.default(as.character(item))
+}