_sanitize_column now reports proper duplicate error
Merge branch 'issue_7432_resolve' of https://github.com/roblevy/pandas into roblevy-issue_7432_resolve
ERR: fixup exception reports for duplicate axis reindexing, related (GH7432)
WIP: categoricals as an internal CategoricalBlock GH5313
Categorical: add new constructor mode
Catgoricals: implement min/max/sort
CLN: integrate JanSchulz changes
     change _value_pointer back to labels
BUG: fix up PeriodIndex formatting/printing
API: Categorical sort api backwarks - now conform to Series interface (order returns a copy, sort is inplace)
ENH: delegate _reduction and ops from Series to the categorical
     to support min/max and raise TypeError on other ops (numerical) and reduction
more min/max tests for categorical as a pd.Series
Add Categorical Properties to Series

's.levels' and 's.ordered' are passed through to the underlying
Categorical instance.
Default to 'ordered' Categoricals if values are orderd

R defaults to ordered factors if the input values are ordered. Do
the same. Change the unittests accordingly.

Also reordere some unittests: catgoricals tested as is and tested as
Series/Dataframes belong to different classes.
Test that Series.ordered/.levels only works on Categorical types
Categorical: add level assignments and reordering + changed default for ordered

Add the possibility to change the levels (including longer and shorter levels).

Add a `Categorical.reorder_levels()` method. Change some naming in `Series`,
so that the methods do not clash with established standards and rename the
other categorical methods accordingly.

Do some refactorings and use a faster version in case values + levels are
passed in at creation time.

Also change the default for `ordered` to True if values + levels are passed
in at creation time.
Initial doc version for working with Categorical data
Categorical: add Categorical.mode() and use that in Series.mode()
Categorical: Rename fast_path to fastpath
Categorical: Use fastpath in some cases
Categorical: Add mode testcases for Categorical
Categorical: implement remove_unused_levels()

Both implemented in Categorical and Series