ERR: disallow direct tz setting
related to #3714
tz should not be settable

```
df=pd.DataFrame({"events":np.ones(len(dates))},index=motor["timestamps"].astype("datetime64[s]"))
df.describe()
```

|  | events |
| --- | --- |
| count | 2393 |
| mean | 1 |
| std | 0 |
| min | 1 |
| 25% | 1 |
| 50% | 1 |
| 75% | 1 |
| max | 1 |

```
df

<class 'pandas.core.frame.DataFrame'>
DatetimeIndex: 2393 entries, 2013-05-30 11:58:36 to 2013-06-03 13:30:43
Data columns (total 1 columns):
events    2393  non-null values
dtypes: float64(1)


df.index.tz='Asia/Kolkata'
df.index

<class 'pandas.tseries.index.DatetimeIndex'>
[2013-05-30 17:28:36, ..., 2013-06-03 19:00:43]
Length: 2393, Freq: None, Timezone: Asia/Kolkata

df2=df[df.index.day==3]

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-39-23de8e1402be> in <module>()
----> 1 df2=df[df.index.day==3]

/usr/local/lib/python2.7/dist-packages/pandas/tseries/index.pyc in f(self)
 39             utc = _utc()
 40             if self.tz is not utc:
---> 41                 values = self._local_timestamps()
 42         return tslib.get_date_field(values, field)
 43     f.__name__ = name

/usr/local/lib/python2.7/dist-packages/pandas/tseries/index.pyc in     _local_timestamps(self)
399             values = self.asi8
400             indexer = values.argsort()
--> 401             result = tslib.tz_convert(values.take(indexer), utc, self.tz)
402 
403             n = len(indexer)

/usr/local/lib/python2.7/dist-packages/pandas/tslib.so in pandas.tslib.tz_convert     (pandas/tslib.c:20949)()

/usr/local/lib/python2.7/dist-packages/pandas/tslib.so in pandas.tslib._get_deltas (pandas/tslib.c:22754)()

/usr/local/lib/python2.7/dist-packages/pandas/tslib.so in pandas.tslib._get_utcoffset (pandas/tslib.c:11327)()

AttributeError: 'str' object has no attribute 'utcoffset'
```

A temporary fix which i did was to add seconds to data manually while creating the index. 
