Frequency alias bug? MS -> millisecond or MonthBegin?
Likely take me a while to work my way through this again completely. Maybe I'm just misremembering this code, but should MS map to millisecond or month begin? (I was looking for a function to easily and generally say that M, MS, M-Jan, M-Dec, etc. are all monthly frequencies.)

```
pd.tseries.frequencies.get_standard_freq('MS')
pd.tseries.frequencies.get_freq_group('MS')
```

vs. things like

```
pd.tseries.frequencies.get_offset('MS')
```

and all the rest of the code that gives freq and freqstr MS for month start code like.

```
pd.tseries.offsets.MonthBegin().freqstr
```
