small changes for compatibility with SMF 2.0.x

This PR adds minor changes to make ajaxChat 0.8.7 compatible with SMF 2.0.x

- SMF uses a unique prefix on all its tables so we need to add it to our ajax chat tables as well.
- Since we already added the prefix, the check on AJAXChat::getDataBaseTable() becomes obsolete so re-write the method to skip it.