Better freqstr for non-Tick frequencies with offset
```
In [1]: date_range('1/1/2000', periods=10, freq='1B30min')
Out[1]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2000-01-03 00:30:00, ..., 2000-01-13 04:30:00]
Length: 9, Freq: B, Timezone: None

In [2]: date_range('1/1/2000', periods=10, freq='1B30min').offset
Out[2]: <1 BusinessDay: offset=datetime.timedelta(0, 1800)>
```
