rolling_* function timeseries usage
here is my problem.
What I have is a DataFrame as follows:

```
df:

2013-10-24      1
2013-10-25      2
2013-11-27      3 
2013-11-28      4
2013-12-01      5 
2013-12-02      6
```

What I want is a DataFrame such like this:

rolling_mean(df, window='1M'):

```
2013-10      1.5
2013-11      3.5
2013-12      5.5 
```

rolling_mean(df, window='2M'):

```
2013-10      NAN
2013-11      2.5
2013-12      4.5 
```

rolling_mean(df, window='3M'):

```
2013-10      NAN
2013-11      NAN
2013-12      3.5 
```

rolling_mean(df, window='1Y'):

```
2013-10      NAN
2013-11      NAN
2013-12      NAN
```

where 1M is '1 month', 2M is '2 months'. The window is not a int value, but a time interval such as '1M', '3M', '1Y' and so on. The function could groupby the dataframe by the time unit such as 'M', 'Y', and then rolling the dataframe through the number before the time unit such as 1, 3...

I need a rolling function such like this?
Could anybody help me? Did I give a clear description?
Many thanks.
p.s.
The new dataframe's Freq is the time unit such as 'M' or 'Y', which is not related to the number.
