Update to 2.020

Upstream changes:
2.020    2013-01-20

    - Give a more informative error message when a PDF file using a
      cross-reference stream is encountered.  The Known Issues section
      of the documentation includes pointers on how to add support in
      case someone else can get to this before I do.

    - Text using TrueType fonts is now searchable again.  In version
      0.61, an undocumented -unicodemap option was added to nearly all
      of the font resource methods, which only included a ToUnicode
      CMap if it was set (and it's required in order for PDF readers
      to be able to identify individual characters if BaseEncoding
      isn't present, which it isn't for TrueType fonts).

      I've left the -unicodemap option in place, and it's still
      undocumented (except here), but it's now on by default.  Call
      $pdf->ttfont($fontfile, -unicodemap => 0) if you want to disable
      it for performance or file size reasons.

    - Add a note to the stringify method's documentation saying that
      it's a destructive operation.

    - The various filter types have their own classes in the
      PDF::API2::Basic::PDF::Filter namespace now, rather than having
      all of the packages in Filter.pm.

    - To facilitate testing and ensure that identically-generated PDFs
      have identical output in Perl 5.17.5 or later, PDF dictionary
      keys are now sorted during output.

(wen)