Refine API/framework core functionality

I haven't yet reached this task (see #833642-7: Create admin interface), but here is already a list of things that should be done here, before implementing a concrete service class:

• Implement hook_entity_info() for indexes and servers.
• Restructure the class structure:
  • Entity classes for servers and indexes
  • Service class is created by server entity class, calls are delegated
  • Queries are created with $index->query(), server can be accessed with $index->server(), etc.
  • Search is done completely with query object, takes care of pre-/postprocessing and calls server method (see other query classes, e.g. for database or EntityFieldQuery)
• Create an exception class and throw those when encountering wrong input (instead of watchdog(…); return FALSE;
• Slightly change dataflow when indexing, entity metadata wrappers should be used
• Make API language-aware! (Language(s) should be set on an index, and are indexed seperately. Some kind of "content negotiation" when searching.)

The following tasks are now completed:

• Implement hook_entity_info() for indexes and servers.
• Restructure the class structure:
  • Entity classes for servers and indexes
  • Service class is created by server entity class, calls are delegated
  • Queries are created with $index->query(), server can be accessed with $index->server(), etc.
  • Search is done completely with query object, takes care of pre-/postprocessing and calls server method (see other query classes, e.g. for database or EntityFieldQuery)
• Create an exception class and throw those when encountering wrong input (instead of watchdog(…); return FALSE;

Up next is using entity_metadata wrappers to retrieve fields and data, and then to make the whole thing language-aware.

There were some more things still to do that I didn't think of earlier, but now the core functionality is pretty sound, I think.
A bit of fiddling will still be done while implementing the concrete backend, but the main work here is done for now.