[feature]: expand basic-price-oracle example so that more advanced testing can be conducted

[ZZiigguurraatt]

basic-price-oracle has the asset id, buy rate, and sell rate hard coded into it. So, one needs to re-compile every time they want to test with a new asset and also when the exchange rate changes. This is totally impractical for automated testing where new assets are created every time a new test network is created and it does not allow us to test with volatile exchange rates.

We need some way to adjust these parameters. Maybe it is with command line options, maybe it is with a config file that is automatically reloaded when the file is re-written, maybe it is from another RPC call that accepts new values, maybe it is by allowing the price oracle to be a proxy for other existing exchange rate sources such as https://www.bitstamp.net/api/ticker/btcusd . Maybe it is all of these things.

We also need to be able to change the server listen address so that we can have multiple price oracles running on the same machine at the same time. This would allow us to operate a price oracle for multiple assets without needing to make the basic-price-oracle example support multiple assets. For the same reason, if we do have a config file, we should be able to change the path for the config file as a command line option.