Added Map[K, V].andThenM(Map[V, W]): Map[K, W], closes #45
Added Set.{mutable, toMutable}, Map.toMutable, closes #109
Added 'directory' parameter to File.create
Added File.deleteRecursively
File.withTempDirectory deletes recursively
Added File.deleteRecursivelyOnExit
File.tempDir deletes recursively on exit, closes #106

Refactored slightly to reduce number of shutdown hooks created
  tempDir is NOT implemented in terms of tempFile because the
    latter, presumably, registers a shutdown hook when it calls
    'deleteOnExit'
  withTempFile & withTempDirectory are NOT implemented in terms of
    tempFile & tempDir because the former both deletes in finally
    blocks