Allow config/cequel.yml to specify replication and durable_writes settings

The rake task cequel:keyspace:create does not provide a method to pass in options when creating the keyspace.

This adds support to replication and durable_writes settings on per environment basis.

@becomingwisest proposed #152 but it's missing durable_writes option and tests.
Providing a `:replication` option that takes the full replication configuration seems right to me, but I don't want to *remove* any options, since that's a breaking change. I think what we should do here is look for the `:replication` key, and then if that's not provided, check for `:class` and `:replication_factor` and build the `default_replication_options` from those if they're provided. Probably would make sense to just raise an error saying to use the `:replication` option if a `:class` option other than `SimpleStrategy` is given, since it's not possible to configure a `NetworkTopologyStrategy` without providing the full `:replication` hash.