WARNING: You have to clear the cache after this update.

Main results:
- Completed search_api_db module.
- Added HTML filter preprocessor.
- Wrote tests.
- Several bug-fixes, etc.

- Fixed entity bug and furthered changes.

* table search_api_backend -> search_api_server (and all corresponding uses of "backend")
* hook_search_api_backends() -> hook_search_api_service_info() (and all corresponding uses of "backend")
* return array of hook_search_api_service_info() not keyed by class name any more (provides greater flexibility for _alter hooks)