Support for {'$meta': 'textScore'} in sort for mongo 2.6

[dkliminsky]

something:

index = SON()
for (key, value) in index_list:
    if not isinstance(key, basestring):
        raise TypeError("first item in each key pair must be a string")
    if value not in [ASCENDING, DESCENDING, GEO2D, {'$meta': 'textScore'}]:
        raise TypeError("second item in each key pair must be ASCENDING, "
                        "DESCENDING, or GEO2D")
    index[key] = value
return index