Add setBucket function

Hi,

I added the support to libmemcached vbuckets function, making the match to memcached_bucket_set function:

LIBMEMCACHED_API
  memcached_return_t memcached_bucket_set(memcached_st *self,
                                          const uint32_t *host_map,
                                          const uint32_t *forward_map,
                                          const uint32_t buckets,
                                          const uint32_t replicas); 

'host_map' needs to have 'buckets' elements;
'forward_map' needs to have 'buckets' elements or NULL;

I made tests writing keys using libmemcached and reading using php-memcached and vice-versa.