Indexing/index settings improvements:

- Add support for custom index settings in config (updates without full reindex).
- Flag index dirty instead of reindexing immediately after product import.
- Skip delete operations for disabled stores.
- Fix broken clean index.
- Remove unnecessary resetSearchResults (caused huge database load after
  mass actions).
- Make indexing product count optional.
- Do not full reindex on product/category delete.
Fix foreign key errors related to deleted products.
Merge pull request #49 from colinmollenhour/indexing

Indexing/index settings improvements: