fix activity import oid sort issue
fix #114; dropped dependency on (non-mongodb provided) bson module, which causes problems when installed alongside pymongo, which installs a custom bson module (named also, bson, doh!); see http://stackoverflow.com/questions/17266010/python-django-mongodb-importerror-no-module-named-bson-objectid#comment28220211_17272710; run `pip remove bson; pip install -U pymongo` to cleanup; metrique supports pymongo 2.2+ only, which means we should always expect bson.objectid.ObjectId not pymongo.objectid