dynamic elements when adding rows

# see discussion in issue 572. Copy of the code example:
```
data = tablib.Dataset()

txt='''Greg Thorton
Dave Coutts
Joshua Ourisman'''

data.extend([item.split() for item in txt.splitlines()])

def dynamicColumn(row):
return row[-1].upper()+'!'

data.append_col(dynamicColumn,'Calculated')

data.append(('Brad','Montgomery', dynamicColumn))
```