Fix: Add compat code for pd.Categorical in pandas>=0.15

pandas renamed pd.Categorical.labels to pd.Categorical.codes. It's
also now possible to have Categoricals as blocks, so Series can contain
Categoricals.
@JanSchulz you might want to define something like: ``is_pandas_cat_support = LooseVersion(pd.__version__) >= 0.15.0``