Update quickbooks.py

This can now be used stand alone if you just want to get the QBO v3 objects from the API. You can grab the companion massage and report modules (which this will import if they're there) for functions that require line-level views of this data. There are functionality gaps, mostly where Intuit hasn't implemented certain kinds of transactions (such as bank deposits), but also where I haven't implemented a ledgerization technique (such as for VendorCredit objects).