replaces #1591 and #1592 , closes#1571 Replaces the Currencies.txt to a json format which includes the networks too ( there is also a test to check now) Also makes CurrencyPair not depend on network and instead use CurrencyNameTable
The CoinGecko rate provider is similar to the bitcoin average one, in that you can ask it for a rate from its aggregated sourcing or you can get rates from specific exchanges. I've added support for both.
I haven't integrated it or replaced coinaverage just to see if we should use it as the default and switch everyone to it or what other action to take.
