CHANGES IN corrplot VERSION 0.76

• In corrplot(), added parameters na.label and na.label.col that define how NA values
  inside a matrix should be rendered.
• In corrplot(), na.label can now use one or two characters, default is "?" (issue #55)
• Fixed #17: restore par() settings on exit (thanks, @yihui)
• Fixed #16: checks for [-1, 1] interval are too strict.
• Fixed #15: error with correlation plot using insig argument when all values are significant.
• Fixed #9: added ward.D and ward.D2 hclust methods (thanks, @jeffzemla)