Allow /rates API to take in a base parameter

[mkurapov]

Currently, there is no way for the ASE to specify the base currency for the /rates API.
In the MASE, we are returning an object like:

    {
      base: 'USD',
      rates: {
        EUR: 1.1602,
        ZAR: 17.3792
      }
    }

However, the base would need to be different based on what is passed in the /rates API, eg:
http://cloud-nine-wallet/rates?base=EUR would return

    {
      base: 'EUR',
      rates: {
        USD: 0.8619,
        ZAR: ...
      }
    }

make sure to update the code to the MASE to handle this new parameter as well.