Better row interface

[JovanVeljanoski]

It would be very helpful if df.expression supported:

• slicing, (e.g. df.x[:10] and df.x[:10].values)
• argmin and argmax, (e.g. df.x.argmin() and df.x.argmax())

Perhaps it would be also nice to have support for argsort as a whole, although that would be quite intensive for large datasets. The use-case I see coming up is the need to find the n smallest or largest indices of a particular expressions, so those rows in the DataFrame can be queries or inspected in more detail.