sqlite support

This patch enables basic support for using sqlite instead of mysql for an ImpressPages site. It does not support installing IP into sqlite yet, but you can run (and edit) the site from a converted database (using http://www.sqlabs.com/sqliteconverter.php).

Why should one want to use sqlite instead of mysql for an IP website? Some reasons:

* during development, you can keep all changes in git/mercurial (since sqlite db is just a file)
* deploy via ftp/scp/rsync only, no need to transfer db
* for low traffic websites, sqlite can be faster and use less memory