Making each_row&each_instance methods more concise

[RaVbaker]

Now it's going to use the block variable from method and it's using options argument for each_row call.

It fixes #6

[afair]

Thanks for catching the buffer_size/block_size confusion.

Actually, the option should be called :batch_size for uniformity with find_in_batches(), but I don't want to create a migration issue with this. I think I will try to make them (eventually) consistent. :-)