Upgrade TCPDF to 6.0.023

Changes:

6.0.023 (2013-08-05)
    - GNU Freefont fonts were updated.
    - Licensing and copyright information about fonts were improved.
    - PNG image support was improved.

6.0.022 (2013-08-02)
    - fixing initialization problem for signature_appearance property.

6.0.021 (2013-07-18)
    - The bug caused by the preg_split function on some PHP 5.2.x versions was fixed.

6.0.020 (2013-06-04)
    - The method addTTFfont() was fixed (Bug item #813 Undefined offset).

6.0.019 (2013-06-04)
    - The magic constant __DIR__ was replaced with dirname(__FILE__) for php 5.2 compatibility.
    - The exceptions raised by file_exists() function were suppressed.