Add OANDA and ECB importers, update with fallthrough

exchange_rates_cron can now go through a list of retrievers, stopping
when it gets a good result.  The OANDA retriever uses OANDA's new API.
Users can configure which data point to retrieve (high/low/average bid,
or the midpoint between average bid and average ask).
The fallback ECB update logic has been adapted into a standalone
retriever.  A few tests for the OANDA retriever are included.

Change-Id: I4dad864e210480fd963a2bdaec854567de632a3d
Merge "Add OANDA and ECB importers, update with fallthrough"