'infer_freq' can return timerule (WOM-5MON) that 'to_offset' does not support
``` python
import pandas as pd
index = [pd.Timestamp('2014-03-31 00:00:00'),
         pd.Timestamp('2014-06-30 00:00:00'),
         pd.Timestamp('2015-03-30 00:00:00'),
         pd.Timestamp('2015-06-29 00:00:00')]
freq = pd.infer_freq(index)  # WOM-5MON
pd.datetools.to_offset(freq)  # ValueError: Could not evaluate WOM-5MON
```

Instead of supporting WOM-5###, I think we can easily modify _get_wom_rule() to return None for this set of dates so it doesn't get inferred.  Agree?
