Support for private date (issue #61)

[outis]

Mostly, these commits are about supporting private information from the BGG API (issue #61). There are a few with other updates: a new XML utility function (xml_attr) in the spirit of others, tests for loaders/collection.py and a bugfix for add_collection_items_from_xml. It also includes a fix for issue #67 (which is also accomplished by pull #68), but that was only so the tests wouldn't get stopped by it.