PhpWiki 1.6.4 released

This is a bug-fix release.

Fixes:
* Check for "shell_exec" before using it (Christof Meerwald)
* Improve RSS (Christof Meerwald)
* Add support for SQLite in PDO, add support for SQLite3 in PEAR (Christof Meerwald)
Changes:
* Upgrade PEAR to release 1.10.14, PEAR DB to release 1.12.1