Support for Swift v4

[marbetschar]

This pull requests add support for Swift v4 in FX. Please note, my main goal was to get this working again - not to provide a fully overhauled API. That said, folks can at least use FX again in their projects and we got a new common ground to build upon.

This pull requests depends on danthorpe/Money#92 - which adds arithmetic support for ISOMoney when used together with Decimal.

I've added a new protocol ISOMoneyProtocol which to allows a FXProvider to work with all available currencies (ISOMoneyProtocol bridges the needed values from ISOMoney<C: ISOCurrencyProtocol> in a generic way). You may or may not want to move ISOMoneyProtocol to the Money framework instead of FX.

The dependency upon ValueCoding is completely removed as it's obsolete. However, I did not implement the Codeable protocol. Shouldn't be too hard to add it if anyone needs it.

The Yahoo Provider is completely removed too, since the service is no longer available. For details see: https://stackoverflow.com/questions/47099005/did-yahoo-finance-just-discontinue-their-csv-download

[marbetschar]

PS: I've also commented the Bitcoin integration, since there does not seem to be support for it in the Money framework anymore - at least I did not found the corresponding currency in the Xcode 9 branch ;)