• our JSS paper referenced
• vignette added
• minor improvements

idendro 1.5.2

• 'plumbr' and 'grDevices' packages must be depended upon (in order to
  let users use their functions directly)

idendro 1.5.1

• version updated to align with that of 'idendr0'
• a bug with "locked environment" in createButtons()" experienced under
  R 3.2.0 fixed
• informative warning()'s turned into message()'s
• several minor bugs fixed