Missing opts={} for trade_volume

Line 108:

``` ruby
def trade_volume(asset_pairs)
  opts['pair'] = asset_pairs
   post_private 'TradeVolume', opts
end
```

Should probably be:

``` ruby
def trade_volume(asset_pairs, opts={})
  opts['pair'] = asset_pairs
   post_private 'TradeVolume', opts
end
```
