Changes in .ix behavior that break backwards compat
It seems like there was major refactoring of the .ix[] indexing in 0.13 that breaks backwards compatibility in some ways.

E.g. `.ix[row][col_name] = x` used to work but doesn't. Sometimes `.ix[row, col_name] = x` also doesn't work and I have to use `.loc[row, col_name] = x` instead although the docs state that .ix[] just falls back to either .loc or .iloc.

I'm changing most of my code but I guess I'm just wondering whether that was made explicit as I can't find it in the release notes or if unittests failed to catch it?
