* Created AbstractProvider class (for all DB services), which handles the default batch_size option.
* The logger Closure is now optional for populate().
* Removed unused Elastica_Type argument from Provider constructors.
* Added unit tests for Doctrine's AbstractProvider class.
* The extra argument (ManagerRegistry) for Doctrine providers is now an appended constructor argument, so the extension no longer needs to use different replacement indexes for Propel/Doctrine providers.
