Add getTime(), setRankingModel(), multiple indexes. Update readme

1. Added function getTime() to make it easy to display summary line on search page. Ex.
"retrieved SphinxSearch::getTotalCount() matches in SphinxSearch::getTime() sec."
2. Added setRankingMode() to make it more feature complete
3. Ability to specify multiple indexes (main/delta)
4. Updated readme.md with an example of how to achieve paging and caching with laravel 4 and sphinx. Also added instructions for multiple index search (still maintains backward compatibility)