Don't apply attribute updates to real-time indices.

They're already being updated with their own callback.
Connection error messages now mention Sphinx.

They were already ThinkingSphinx::ConnectionError instances, but the message matters too.
Spec fix for real-time index exclusion in attribute updates.
Tentative support for HABTM attributes with query sources.

It is only useful for when you join on the foreign key - and even then, the DSL syntax is not quite technically right:

    # in index for Book
    has books_genres.id, :source => :query

The resulting join is just on the HABTM table, and id becomes genre_id.