memoise 1.0.0

• memoise() now signals an error if an already memoised function is used as
  input (#4, @richierocks).
• has_cache() function added which returns a boolean depending on if the
  given call is cached or not (#10, @dkesh).
• Memoised functions now have a print method which displays the original
  function definition, rather than the memoisation code (#15, @jimhester).
• A memoised function now has the same interface as the original function,
  if the original function is known when memoise is called. (Otherwise,
  the old behavior is invoked, with a warning.) (#14, @krlmlr)
• The enclosing environment of the memoised function is specified explicitly,
  defaults to parent.frame().
• is.memoised now checks if the argument is a function.
• Testing infrastructure, full test coverage.