code: function get(currency, callback) {
    if(['BTC', 'USD', 'AUD', 'NZD', 'CAD', 'EUR', 'GBP', 'SGD', 'HKD', 'CHF', 'JPY'].indexOf(currency) === -1) {
      throw new Error('specified currency is not supported');
    }
  
    if(!util.isFunction(callback)) {
      throw new Error('callback must be a function');
    }
  
    var resource = this.coinjar.personal.api_endpoint +
      '/v1/fair_rate/' +
      currency +
      '.json';
  
    request.get(resource)
      .auth(this.coinjar.personal.auth_token)
      .end(function (response) {
        if(isResponseError(response, callback)) { return; }
        return callback(null, response.body);
      });
  }
docs: @name get
  @description
  Fair Rate for currency.
  
  @params {String} currency
  @params {Function} callback