Added function for new Segmentation Api Endpoint multiseg

This is a new api endpoint that Mixpanel has not documented yet. It allows segmentation on 2 different fields.
The 'inner' and 'outer' properties are segmentation expressions and are required

example:
mixpanel.segmentation().multiseg(event='purchased_membership', type='general', from_date='2013-05-01', to_date'2013-07-10', inner='os', outer='authentication_type', limit-25)