ENH: tz_localize(None) allows to reset tz

Closes #7812. Allow ``tz_localize(None)`` for tz-aware ``Timestamp`` and ``DatetimeIndex`` to reset tz.
CC @rockg, @nehalecky 

```
t = pd.Timestamp('2014-01-01 09:00', tz='Asia/Tokyo')

# tz_localize resets tz holding current tz representation
t.tz_localize(None)
# 2014-01-01 09:00:00

# tz_convert reset tz using UTC representation (no change)
t.tz_convert(None)
# 2014-01-01 00:00:00

idx = pd.date_range(start='2011-01-01 09:00', freq='H', periods=10, tz='Asia/Tokyo')

idx.tz_localize(None)
# <class 'pandas.tseries.index.DatetimeIndex'
# [2011-01-01 09:00:00, ..., 2011-01-01 18:00:00]
# Length: 10, Freq: H, Timezone: None
 
# (no change)
idx.tz_convert(None)
# <class 'pandas.tseries.index.DatetimeIndex'>
# [2011-01-01 00:00:00, ..., 2011-01-01 09:00:00]
# Length: 10, Freq: H, Timezone: None
```

``tz_convert(None)`` removes timezone, returning an UTC time (see above example). This behavior is NOT changed by this PR, but not documented anywhere.