allow for statistical facet type

Doing something like this:

>>> qq=q.facet_raw(controller={'statistical':{"field":"pta_nm"}})
>>> qq.facet_counts()

Will return something like this:

`{u'controller': {u'count': 314743, u'_type': u'statistical', u'min': -1624.0, u'sum_of_squares': 58189684011.0, u'max': 986.0, u'std_deviation': 353.6764163422847, u'variance': 125087.0074767211, u'total': -76962861.0, u'mean': -244.52604505898464}}`