Period.asfreq broken for freq=MS
This works:

```
In [9]: pd.Period("2013").asfreq(freq="M", how="S")
Out[9]: Period('2013-01', 'M')
```

but this does not:

```
In [8]: pd.Period("2013").asfreq(freq="MS", how="S")
----> 1 pd.Period("2013").asfreq(freq="MS", how="S")

/usr/local/lib/python2.7/dist-packages/pandas/tseries/period.pyc in asfreq(self, freq, how)
    196         how = _validate_end_alias(how)
    197         base1, mult1 = _gfc(self.freq)
--> 198         base2, mult2 = _gfc(freq)
    199 
    200         if mult2 != 1:

/usr/local/lib/python2.7/dist-packages/pandas/tseries/frequencies.pyc in get_freq_code(freqstr)
     99 
    100     base, stride = _base_and_stride(freqstr)
--> 101     code = _period_str_to_code(base)
    102 
    103     return code, stride

/usr/local/lib/python2.7/dist-packages/pandas/tseries/frequencies.pyc in _period_str_to_code(freqstr)
    744         return _period_code_map[freqstr]
    745     except:
--> 746         alias = _period_alias_dict[freqstr]
    747         return _period_code_map[alias]
    748 

KeyError: 'L'
```

it seems to be treating the capital `MS` as lower case `ms`, meaning millisecond. Observer similar results:

```
In [11]: pd.Period("2013").asfreq(freq="ms", how="S")
----> 1 pd.Period("2013").asfreq(freq="ms", how="S")

/usr/local/lib/python2.7/dist-packages/pandas/tseries/period.pyc in asfreq(self, freq, how)
    196         how = _validate_end_alias(how)
    197         base1, mult1 = _gfc(self.freq)
--> 198         base2, mult2 = _gfc(freq)
    199 
    200         if mult2 != 1:

/usr/local/lib/python2.7/dist-packages/pandas/tseries/frequencies.pyc in get_freq_code(freqstr)
     99 
    100     base, stride = _base_and_stride(freqstr)
--> 101     code = _period_str_to_code(base)
    102 
    103     return code, stride

/usr/local/lib/python2.7/dist-packages/pandas/tseries/frequencies.pyc in _period_str_to_code(freqstr)
    744         return _period_code_map[freqstr]
    745     except:
--> 746         alias = _period_alias_dict[freqstr]
    747         return _period_code_map[alias]
    748 

KeyError: 'L'
```
