Issues Creating Period with DateOffset as freq
Something seems wrong here:

```
print repr(pd.Period("2013-12", freq=pd.offsets.MonthEnd()))
```

correctly prints:

```
Period('2013-12', 'M')
```

but:

```
print pd.Period("2013-12", freq=pd.offsets.BusinessMonthEnd())
```

leads to:

```
Traceback (most recent call last):
  File "/home/alex/git/pandas/pandas/tseries/tests/test_52.py", line 29, in testName
    a = pd.Period("2013-12", freq=BusinessMonthEnd())
  File "/home/alex/git/pandas/pandas/tseries/period.py", line 121, in __init__
    base, mult = _gfc(freq)
  File "/home/alex/git/pandas/pandas/tseries/frequencies.py", line 92, in get_freq_code
    code = _period_str_to_code(freqstr[0])
  File "/home/alex/git/pandas/pandas/tseries/frequencies.py", line 757, in _period_str_to_code
    alias = _period_alias_dict[freqstr]
KeyError: 'BM'
```

Strangely, the following both work:

```
In [10]: pd.date_range("2013-01","2013-10",freq=pd.offsets.MonthEnd())
Out[10]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-01-31 00:00:00, ..., 2013-09-30 00:00:00]
Length: 9, Freq: M, Timezone: None

In [11]: pd.date_range("2013-01","2013-10",freq=pd.offsets.BusinessMonthEnd())
Out[11]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-01-31 00:00:00, ..., 2013-09-30 00:00:00]
Length: 9, Freq: BM, Timezone: None
```
