[#77] Access counts are incremented synchronously.

Undoing premature optimization.
[#77] Remove category.name from the list of analysed fields.

Analysis of associations should be implemented with a bit more thought.
Analysing the name of the category for articles does not give
significantly better search results, so can be safely removed.

This also allows us to model#send rather than model#instance_eval
[#77] Only analyse articles when relevant fields are updated.

* Prevents analysis when access_count is incremented.
* Prevents analysis when article saved with no changes.
[#77] Delete relevant cache key instead of blanket cache.clear