Follow-up to issue #1064884 by drunken monkey, Damien Tournoud: Fixed hook_enable(), hook_disable() and some documentation and comments.

Follow-up to issue #1123604 by fago: Fixed update hook for generalized "aggregation" data alteration.

#1078590 by fago: Added check to skip default index creation when installed via installation profile.

* 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)

Expanded functionality, implemented functions, added some hooks and did some refactorization (but still more planned). Bit of a huge commit, I guess.

Some change in terminology and naming, more are planned.