JCBC-630: Add toString to Bucket

Motivation
----------
When logging, it is more interesting to see significant information on
a Bucket than just its memory reference (default toString).

Modifications
-------------
Added toString overload for CouchbaseBucket and CouchbaseAsyncBucket.
The String representation is in the form "Bucket[<name>]" and
"AsyncBucket[<name>]".

Result
------
Logged Buckets now display meaningful information.

Change-Id: I8addce1e453f9effa578c822456e685753712629
Reviewed-on: http://review.couchbase.org/43357
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Simon Baslé <simon@couchbase.com>