Fix reindexing rake tasks, don't reindex when initializing search

We'd been waiting to configure Sunspot on our models until the first time a SearchEngine was used. This made all of Sunspot's built-in reindexing tasks fail without doing any actual reindexing.

Secondly, we were reindexing when setting up search, which caused a huge performance hit on the first search performed after startup (especially on a large database).

Fixes #138