Support standard partition assignment strategies

[emmettbutler]

#655 points out pykafka's lack of support for the standard roundrobin and range partition assignment strategies, described here. Support for these strategies is required for interoperability with other kafka clients. The strategy should be selectable by a kwarg on BalancedConsumer that will accept a string roundrobin|range and will effectively swap between different implementations of _decide_partitions.

[jeffwidman]

Are you sure you want the kwarg to be a string? There are more partition strategies coming down the pipe, for example KIP-54 is slated for Kafka 0.11.0. Plus a user may wish to implement a custom one. So you might want to make the kwarg be a subclass of a basic partitioning class, with the two shipped by default being RoundRobin and Range