ENH: let get_dummies take a DataFrame

implement via 1d

fixup docstring, tests

add documentation

test for dicts
DEPR: deprecate convert_dummies

add depr to doc
Merge pull request #8140 from TomAugspurger/df-dummies

ENH: let get_dummies take a DataFrame
API/FIX: wrap result cut/qut in Series and make it ordered

As per the discussion in #8077, the result of cut/qcut should be
wrapped into a Series, now that this is possible.

Also change the returned Categorical to "ordered=True", this was
overlooked when this was introduced in Categorical.

Closes: #8077
FIX: Categorical.unique() should return an array

As per dicussion in https://github.com/pydata/pandas/issues/8074,
Categorical.unique() should return an array and not a Index object
to make is consistant with other unique() implementations.
DOC: Mark a few more categorical methods as internal
API: Implement Series.cat.codes

Series should also expose Series.cat.codes as a Series of codes.

See discussion in
https://github.com/pydata/pandas/issues/8074#issuecomment-53992337
API: remove compat keyword in Categorical constructor

Before the new Categorical work, the default two argument constructor
was expecting "codes and levels" but this was changed to
"values and levels" and a 'compat' kwarg was added, which could
be used to switch to the old style constructor useage.

It was decided that we switch to the new style constructor useage as
the new default (compat=False) and implement a 'from_codes(...)'
constructor.

As the compat codepath is now never triggered and code should be
changed to use the new 'from_codes()' constructor, remove the old
codepath.

Add some warnings if we are pretty sure that the old style
constructor is meant, but unfortunately we don't catch all cases.