Create a currencyConverter service that updates the price of BTC
Add two dependencies: request and async
Requests for cryptocurrency exchange rates now done on server side

* HTML template now renders each currency with ngRepeat directive
* Angular code written so as not to be coupled to certain currencies
Use Angular currency filter when rendering total value of portfolio
Change API endpoint so only one request is needed for all currencies
Remove dependency: async
Server gets exchange rates every 60 secs, not on each client request
Extract exchange rate update code into seperate file