bucket flush needs to be added

For a given bucket, there needs to be an easy way to invoke the bucket's RESTful flush(). This should be at the bucket level and authentication level. In other words, for a given bucket that I'm doing set()/get() with, I should be able to flush(). That flush() must use the REST interface for the bucket, not broadcast the libcouchbase/memached flush operation.

Note that it is safe for other client instances to be connected during this RESTful flush() operation. Also, it may be worth mentioning in the docs that flush is not atomic on the cluster and will propagate asynchronously.