No SQLite/PostgreSQL support

[rgarner]

Have tried to avoid mySQL-specific syntax but it seems I've succumbed to it in the :related_tags association in SemanticallyTaggable::Tag. This uses a two-row INSERT to represent the inverse relation between tags. However, SQLite doesn't support it.

[rgarner]

Since I've succumbed above, I've also used an INSERT IGNORE in the transitive closure. Anyone wishing to add support for other DBMSs will have to get around two doses of my race to release ;)