Allow Enums in place of certain parameters values
I have a suggestion for how to use Enums to codify less transparent parameter values. Since 3.4, Python comes with built-in support for Enums, which could be useful in certain instances for Pandas.

One example is allowing enums in place of integers for axis values, which frankly to this day I cannot seem to remember which is which. These could for instance be stored in an attribute in each function. This could potentially work like this:

    import pandas as pd

    df = pd.DataFrame({'a': [1,2,3], 'b': [2,3,4]})
    df.sum(axis=df.sum.axis.COLUMNWISE)

    a    6
    b    9
    dtype: int64


Another example is having support for enums in place of string values. This would be useful as a way to codify all supported string values outside of documentation, and also enabling autocomplete support in REPLs. Another example:

    import pandas as pd
    from pandas import axis

    df = pd.DataFrame({'a': [1,2,3,5], 'b': [2,3,4,None]})

    df.sort_values('b', na_position=df.sort_values.na_position.FIRST)

       a    b
    3  5  NaN
    0  1  2.0
    1  2  3.0
    2  3  4.0
