Patching exchange rate data

Now fetching EUR and USD from BTC-E, with a USD fallback to
vircurex, and only adding Yahoo! converted EUR data if we
fail to get it from BTC-E.  This way, we should see consistent
EUR and USD from somewhere theoretically.  Also updated vircurex
url.  This is a massive hack until we get better structure
for exchange rate fetching.  Closes #29 for now.