The function downloads data provided by Kenneth
  French at
  http://mba.tuck.dartmouth.edu/pages/faculty/ken.french/data_library.html.
The download file gets a date prefix (current date in
  format YYYYMMDD) and is stored in directory
  dest.dir. Before any download is attempted,
  the function checks whether a file with today's
  prefix exist in dest.dir; if yes, the file is
  used.
In the original data files, missing values are
  coded as -99 or similar. These
  numeric values are replaced by NA.
Calling the function without any arguments will print
  the names of the supported datasets (and return them insivibly).