* Distinction and interaction between service class and server object
          o Common server properties like id, name, machine name, etc., are now only stored in the server object.
          o The service object has now a defined constructor, which contains the corresponding server object as a parameter.
          o The abstract service class saves $server and a reference to $server->options (as $options) in its constructor.
          o When a service class wants to access other server properties (apart from its own settins), e.g. the server ID, it should just use $this->server->id.
    * Service configuration form handling
          o The signatures of the three SearchApiServiceInterface::configurationForm*() methods have changed.
          o $prefix is gone, simply use form_error() instead.
          o SearchApiServiceInterface::configurationFormSubmit() has a new purpose, as it is now responsible for storing the new service options itself. This is most easily done (when extending the abstract service class) by setting $this->options appropriately.
          o If you haven't overridden these methods, you can ignore these changes – the abstract service's default implementation works just as before.
    * Renamed SearchApiServiceInterface::view() to SearchApiServiceInterface::viewSettings().

Documented Entity API special properties "module" and "status".

Added another test query, with negation and "OR"ing. That one still fails, because of an unknown bug.

Some other small changes and additions.

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