BUG: DTI.value_counts doesnt preserve tz
Found 2 problems related to `value_counts`.
- `DatetimeIndex.value_counts` loses tz.

```
didx = pd.date_range('2011-01-01 09:00', freq='H', periods=3, tz='Asia/Tokyo')
print(didx.value_counts())
#2011-01-01 00:00:00    1
#2011-01-01 01:00:00    1
#2011-01-01 02:00:00    1
# dtype: int64
```
- `PeriodIndex.value_counts` results in `Int64Index`, and unable to drop `NaT`.

```
pidx = pd.PeriodIndex(['2011-01-01 09:00', '2011-01-01 10:00', pd.NaT], freq='H')
print(pidx.value_counts())
#  359410                 1
#  359409                 1
# -9223372036854775808    1
# dtype: int64
```
