Store resultsfrom Google API into PurchaseInvoice object

[tillmo]

The method read_and_transfer of class PurchaseInvoice, see purchase_invoice.py, should be modified, such that as the first argument, instead of a pdf (as now), alternatively, a json with the results from Google can be passed. The json should be parsed into a dictionary, and the items from the dictionary should be stored in an object of class PurchaseInvoice, cf. method parse_krannich in purchase_invoice.py (but this method does a complicated analysis, which hopefully we do not need any longer, because Google has delivered a dictionary with all necessary ingredients already).