COMPAT: pandas private API

[jreback]

So making some changes here that could potentially break some of the de-facto private API: pandas-dev/pandas#13147

IOW if doing:

from pandas.core import common as pdcom

pdcom.is_categorical()
for example. This wont' break now as I left it in the namespace, but we are migrating to
`pandas.api for these types of things. However we won't be able to issue a deprecation warning probably to the next version.

[llllllllll]

Thank you for the heads up, we do have master of pandas in the allowed failures section of our travis matrix so hopefully we can find and correct those deprecation warnings as soon as possible.

(looks like it is failing that build right now for other reasons ;_;)