frequency of time series read with `read_csv`
I f I read a regular time series (with a fixed frequency) with read_csv, the resulting DataFrame has no freuqency.

``` python

In [82]: lines = ['2012-07-27T13:%02d, %d' % (min, index) for index, min in enumerate(range(1, 59))]

In [83]: input = StringIO.StringIO('\n'.join(lines))

In [84]: df = pd.read_csv(input, parse_dates=True, index_col=0, names='A')

In [85]: df.index
Out[85]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2012-07-27 13:01:00, ..., 2012-07-27 13:58:00]
Length: 58, Freq: None, Timezone: None
```

What I would like to have, is a possibility to set the frequency without resampling (or maybe read_csv could guess a frequency). 
