Term meta improvements

Several improvements to the our term meta polyfill. Maybe we should consider splitting this into it's own repo?

Here's what I fixed

- [x] Code formatting and PHPDocs
- [x] The `CREATE TABLE` query now does a `CREATE TABLE IF NOT EXISTS` so as to avoid a warning if the table does actually exist
- [x] Instead of manually specifying `ENGINE=MyISAM` the `CREATE TABLE` query now specifies ``ENGINE=`DEFAULT` `` so that the table is created with the default storage engine.
- [x] Up to now we haven't been deleting any term meta when the associated term was deleted, I've fixed this oversight by hooking into the `delete_term` hook.