Inconsistency between read_csv(... parse_dates) and read_csv followed by astype('datetime')
Write time-zones out in csv file

Datetimes are written to csv in readable UTC. Using `pd.read_csv('file.csv', parse_dates=['date'])` expects UTC. `astype('datetime64')` expects readable dates (with no timezone specified) to be local (which also makes sense). But it leads to the following:

``` python
    import pandas as pd
    from StringIO import StringIO

    df = pd.DataFrame({'date' : [0]})
    df['date'] = df.date.astype('datetime64[s]')
    print df.date[0]

    s = StringIO()
    df.to_csv(s)
    s = s.getvalue()
    print 'csv:'
    print s


    with_parse_dates = pd.read_csv(StringIO(s), parse_dates=['date'])
    print 'parse_date:', with_parse_dates.date[0]


    with_astype = pd.read_csv(StringIO(s))
    with_astype.date = with_astype.date.astype('datetime64')
    print 'astype    :', with_astype.date[0]


    with_astypeZ = pd.read_csv(StringIO(s))
    with_astypeZ.date[0] += 'Z'
    with_astypeZ.date = with_astypeZ.date.astype('datetime64')
    print 'astype w/Z:', with_astypeZ.date[0]
```

output (for me):

```
1970-01-01 00:00:00
csv:
,date
0,1970-01-01 00:00:00

parse_date: 1970-01-01 00:00:00
astype    : 1970-01-01 08:00:00
astype w/Z: 1970-01-01 00:00:00
```

Would be easily fixed by indicating UTC in the csv output, possibly with just the `'Z'` suffix.
