Add SQLite3 support

This commit adds support for installing/running SMF on an SQLite3 database. It currently does not support upgrading from 2.x to 3.x, but that functionality can be added later if needed.

Note that I haven't tested everything with this, but none of the actual SQL changed - just the PHP side of things. The basic functionality (installation, clicking around, posting, clearing the error log, etc.) works fine, so I don't expect anything else to break.