Feature request: purely int-index based version of .ix
First of all, thank you very much for the great library!  I really like the scatter plot in the latest release.  I am very new to the library, but I am enjoying it very much.

However, I am really uneasy about the fallback behavior of the `.ix` attribute.  Why can't we have a purely label based version of `.ix` and a purely integer index version of `.ix`?  I want pandas to raise an error when a data frame does not have integer label I pass rather than fallback to integer index based indexing, because "explicit is better than implicit".

I can write a pull request if it is likely to be pulled.  Or maybe you should write it because it is a big API change.
