Adding new attributes to cassandra-server cookbook

I'm creating a wrapper cookbook on the top of your cassandra-server:tarball recipe, and would like to contribute some changes that could also be helpful for the community. I made sure the changes are backward compatible, so there will be no problem with existing cookbooks.

This pull request contains:
1. Added support to more cassandra.yaml and cassandra-env.sh attributes: max_hint_window_in_ms, partitioner, key_cache_size_in_mb, range_request_timeout_in_ms, streaming_socket_timeout_in_ms, index_interval, auto_bootstrap, enable_assertions, xss, jmx_server_hostname.
2. Added new option "auto" to node[:cassandra][:tarball][:url], that will construct the URL correctly based on the chosen node[:cassandra][:version], because before if I changed the node[:cassandra][:version] attribute in the wrapper cookbook, this was not updated in node[:cassandra][:tarball][:url] (since the default version is set to 2.0.x).
3. Updated README with the new attributes.