Fix for memory issues

Recently I had quite big xml to process (about 45MB xml with 190,000 elements originating from root, and lots of other nested elements) but happymapper keeped crashing with so much data due to full memory consumption. I've made some changes in happymapper's codebase so that it doesn't return whole collection of objects but yields it part by part if I set :limit option. Now if I set it to 1000 for instance, memory usage jumps up only by 20% and stays on this level during whole processing.