Batch size in rake task for large database

Add batch size for rake task `geocode:all`.
This may help people who have large database sets.
I decided to add this feature after the question on [stackowerflow#geocoder-ruby-gem-on-demand](http://stackoverflow.com/questions/23012404/geocoder-ruby-gem-on-demand/23012625)
This is my first commit to open source, please correct if I'm wrong. :pray: