Folder iterator performance optimization

See (this comment)[https://github.com/sookasa/box.py/commit/8f59c68e3b60d949b5eab78917d4030946df9815#commitcomment-5748894]
Merge pull request #33 from samkuehn/master

Fix fields api
Update HISTORY.rst
Merge pull request #32 from k8n/patch-1

Folder iterator performance optimization