Make building independent of the Bundle

It's not necessary to use a `Bundle` to initialize a `Cluster` instance, as it doesn't hook in to the server (e.g. Jersey) like Hibernate does.

Instead, it's better to have the `CassandraFactory` itself take the `Environment` and be responsible for managing, adding healthchecks and registering metrics.

I've preserved the `Bundle` API, to ensure existing code works. Also, the `Bundle` API is still the only way to use a `SessionFactory`, though that's largely cosmetic, as you can just call `Cluster#connect(keyspace)` yourself.

Note: tests updated and pass. `README.md` updated to reflect new API.