Add switch-to-new-search command

Add the switch-to-new-search command under the search command. This is
used when migrating from legacy riak_search to yokozuna. It does a
multicall across the cluster handing over HTTP and PB control from
riak_search to yokozuna.
Merge pull request #521 from basho/rz/feature/yz-switch-cmd

Add switch-to-new-search command