Clear Guest Author caches upon Guest Author creation

After a Guest Author is created, we should flush all caches to ensure
any previously cached negative results are gone.

Also need to `$force` the lookup in `delete_guest_author_cache()` to
ensure it never hits a stale negative cache entry.

Fixes #218
Merge pull request #220 from Automattic/fix-218

Clear Guest Author caches upon Guest Author creation