text: Title: CryptoCurrencies.get_digital_currency_exchange_rate doesnt work
  Body: This funtion sends the api call: 
  
  https://www.alphavantage.co/query?function=CURRENCY_EXCHANGE_RATE&symbol=BTC&market=EUR&apikey=XXXX&datatype=json
  
  The right one would be:
  
  https://www.alphavantage.co/query?function=CURRENCY_EXCHANGE_RATE&from_currency=BTC&to_currency=EUR&apikey=XXXX&datatype=json
  
  To fix this the keyword arguments of CryptoCurrencies.get_digital_currency_exchange_rate should be from_currency and to_currency (line 63 in cryptocurrencys.py)
  
  Then the return value should be changed to:
  FUNCTION_KEY, 'Realtime Currency Exchange Rate', None
  
  But im not sure with the Meta_data = None, I just couldnt figure out a secound entry in the dict.
  
  With this changes it works for the default Outputformat but not for output_format = 'pamdas'. Maybe anyone can fix this completely.