memoise 1.1.0

• Caches now hash the function body along with the arguments, to ensure
  functions with identical arguments use a separate file-system cache. (#38)
• Handle missing arguments in memoised functions for simple cases not using
  non-standard-evaluation (#19).
• memoise() gains a cache= argument to specify an external cache. Two types
  of caches are available, cache_s3() for amazon S3 and
  cache_filesystem() for a file system cache (#25, @danielecook).