'Get all subscriptions.
Args:
interval (int): Limit match to tickers with this interval.
Returns:
tickers (list): If `interval` was given, this is a list of
tickers using that interval.
tickerpool_layout (dict): If `interval` was *not* given,
this is a dict {interval1: [ticker1, ticker2, ...],  ...}'

def all(self, interval=None):

    if (interval is None):
        return dict(((interval, ticker.subscriptions) for (interval, ticker) in self.ticker_pool.tickers.iteritems()))
    else:
        ticker = self.ticker_pool.tickers.get(interval, None)
        if ticker:
            return {interval: ticker.subscriptions}
        return None
