Add geocode_raw to yahoo.py for specifying custom flags and getting all results

Occasionally I need either (1) to specify custom flags, or (2) more of the result than just a concatenated location string, longitude, and latitude from yahoo's PlaceFinder API.

This commit adds a method `geocode_raw` to yahoo.py allowing you to do something like this:

    from geopy import geocoders
    y = geocoders.Yahoo('<app_id>')
    r = y.geocode_raw('arlington', flags='JTG')

Which would return a list of matches for 'Arlington" with global formatting and time zone information. Flag descriptions are at http://developer.yahoo.com/geo/placefinder/guide/requests.html#flags-parameter