added MongoDB 2.4 2d indexes

    # Create a new index.
    #
    # @param [String, Array] spec
    #   should be either a single field name or an array of
    #   [field name, direction] pairs. Directions should be specified
    #   as EM::Mongo::ASCENDING, EM::Mongo::DESCENDING, EM::Mongo::FLAT2D, EM::Mongo::SPHERE2D
    #
    #   Note that MongoDB 2.2 used 2d flat indexes and called them geo, MongoDB 2.4 has 2d and 2dsphere indexes
    #   EM::Mongo::GEO2D is kept for backward compatiblity and is creating a flat 2d index