Strange and possibly buggy behaviour when using series.dt with series.apply(strftime)
It seems like Series.dt timezone properties to not interact well with series.apply(lambda x : x.strftime("..")) leading to some odd behaviour. Here is a script demonstrating the problem:

```
import pandas as pd
import logging
import pytz
from pytz import timezone

date_format_string = "%a %b %d %Y %H:%M"

def convert_datetime(x) :
    return x.strftime(date_format_string)

def longer_convert(x, tz = timezone('Europe/London')) :
    return convert_datetime(pytz.utc.localize(x).astimezone(tz))


if __name__=="__main__" :
    rng = pd.date_range('6/6/2011', periods=6, freq='H')
    rng2 = pd.date_range('6/6/2011', periods=6, freq='D')

    series = pd.Series(data=rng, index=rng2)
    series.name = "Original"

    converted_series = series.dt.tz_localize('UTC').dt.tz_convert('Europe/London')
    converted_series.name = "Converted"

    formatted_series = converted_series.apply(convert_datetime)
    formatted_series.name = "Formatted"

    longer_converted = series.apply(longer_convert)
    longer_converted.name = "Using pytz"

    df=pd.concat([series,converted_series,formatted_series, longer_converted], axis=1)
    print df
```

Which yields the output 

```
                  Original                 Converted  
2011-06-06 2011-06-06 00:00:00     2011-06-06 01:00:00+01:00   
2011-06-07 2011-06-06 01:00:00     2011-06-06 02:00:00+01:00   
2011-06-08 2011-06-06 02:00:00     2011-06-06 03:00:00+01:00   
2011-06-09 2011-06-06 03:00:00     2011-06-06 04:00:00+01:00   
2011-06-10 2011-06-06 04:00:00     2011-06-06 05:00:00+01:00   
2011-06-11 2011-06-06 05:00:00     2011-06-06 06:00:00+01:00   

                        Formatted             Using pytz  
2011-06-06  Mon Jun 06 2011 00:00      Mon Jun 06 2011 01:00  
2011-06-07  Mon Jun 06 2011 01:00      Mon Jun 06 2011 02:00  
2011-06-08  Mon Jun 06 2011 02:00      Mon Jun 06 2011 03:00  
2011-06-09  Mon Jun 06 2011 03:00      Mon Jun 06 2011 04:00  
2011-06-10  Mon Jun 06 2011 04:00      Mon Jun 06 2011 05:00  
2011-06-11  Mon Jun 06 2011 05:00      Mon Jun 06 2011 06:00  
```

In which the application of the format string using apply caused the times to revert to UTC (Compare the hours in Formatted vs Converted), whereas I expected to get output similar to the "Using pytz" column, where I have displayed the time in local time with no timezone adjustment.

Is this expected behaviour? Can I format to a string directly through Series.dt?
