[FEATURE] PHP Piwik trackter integration

The PHP API of Piwik is now used to build an image tag that will be
used if JavaScript is not available.

See: http://piwik.org/docs/tracking-api/

Resolves: #11246
[FEATURE] Allow global configuration override

This patch will override the settings from config.tx_piwik with
those provided to the Footer.php user function.

Addtionally the obsolete $tc property is removed and the PHPDoc of
the contentPostProc_output() is improved.

Resolves: #56081