Add row filtering operator

This would allow chaining operations like:

``` python
pd.read_csv('imdb.txt')
  .sort(columns='year')
  .filter(lambda x: x['year']>1990)   # <---this is missing in Pandas
  .to_csv('filtered.csv')
```

For current alternatives see:

http://stackoverflow.com/questions/11869910/pandas-filter-rows-of-dataframe-with-operator-chaining
