BUG: Block/DTI doesnt handle tzlocal properly
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Unable to concatenate `Series` with `tzlocal`

```
s = pd.Series([pd.Timestamp('2011-01-01', tz=dateutil.tz.tzlocal())])
s
#0   2011-01-01 00:00:00+09:00
# dtype: datetime64[ns, tzlocal()]

pd.concat([s, s])
# UnknownTimeZoneError: 'tzlocal()'
```

It is caused by 2 issues:
#### 1. `tslib.maybe_get_tz` can't handle `tzlocal`.

```
pd.tslib.maybe_get_tz('tzlocal()')
# UnknownTimeZoneError: 'tzlocal()'
```
#### 2. `DatetimeIndex.tz_localize` may return incorrect result when `tzlocal` is used.

it's internally used in `conat`.

```
pd.DatetimeIndex(['2011-01-01', '2011-01-02'], tz=dateutil.tz.tzlocal()).tz_localize(None)
# DatetimeIndex(['2011-01-01', '1970-01-01'], dtype='datetime64[ns]', freq=None)
```
