v0.2.1.7

- Fixed problem with garbage in cache after switching currency (the
cache will now be flushed when changing currency)
- Fixed problem fetching trade data with very short intervals and
currecies with very low trading volumes (it's still a problem to get
accurate EMA-values, but it's not quite possible when hardly any trades
are being made!)
- Selecting currency in settings is now made from a drop-down list
- New icon: green eyes = trading is enabled :)