Bug fixes and minor improvements

• set_docinfo_exiftool() should now losslessly write all legal datetimes (#55).
  set_docinfo()'s priority order has been updated to the following:

  1. set_docinfo_exiftool()
  2. set_docinfo_gs()
  3. set_docinfo_pdftk()

• Unit tests have been adjusted to prevent CRAN R CMD check ERRORs on certain CRAN platforms
  that don't have ghostscript installed or are ran in a non-Unicode locale (#56).