BUG Fix old indexing storing against the incorrect class key

Fixes #38

This builds on the work of @DarrenInwood, who correctly added a hook to delete expired documents.

The remaining problem was that the key that a record is stored against uses the class name, whereas when a deletion occurs it attempts to delete it using the base name. E.g. I store a Page, with a record ID "3-Page", and attempts to delete this would identify this record as "3-SiteTree", which would fail.

This fix makes all behaviour consistently use the base class.

Users who upgrade to this version will need to run `./framework/sake dev/tasks/Solr_Reindex "flush=all"` once they have upgraded, as all existing data will be recorded under the wrong key. Upgrading docs to come.