NCBC-698: Add BucketType property to IBucket

User code should be able to distinguish Memcached and Couchbase buckets
without relying on SDK implementation details (calling `bucket.GetType()
== typeof(CouchbaseBucket)`)

Change-Id: Id1efdb895cd5c5f7e80e2e044c19b3f97902d2bb
Reviewed-on: http://review.couchbase.org/42374
Reviewed-by: Simon Baslé <simon@couchbase.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>