DOC/PY3: Python 3 docs example compat for range, StringIO and datetime

related is #6212

Fixes examples in documentation to be compatible with Python 3. Also tested it locally under Python 2.

* Turned occurrences of `(x)range()` to `range()` for iteration and to `list(range())` for  list concat.
* Fixed leading zero argument(s) as in e.g. `datetime(2013, 01, 27)`.
* Removed explicit `StringIO` and `cStringIO` imports in examples (uses suppressed pandas.compat imports) and added a note on py2/3 library reorganization.
* Fixed `StringIO` for binary data to the more explicit `BytesIO`.
* Fixed `NameError` for missing `pd.` in `pd.Series`.
* Added note for failing `numpy.unique` example, see also #6229.  

I am not sure about the `BytesIO` though. Maybe good to put a note saying this is Python 3 way to do it and users on 2.x should use the old `StringIO` instead. In general, should we favor Python 3 doc versions in such cases and add notes for Python 2.x users? What do you think?