Feature/riak cli handoff enable

This adds CLI commands to enable/disable handoffs:

riak-admin <enable | disable> <inbound | outbound | both> [--node <NODE>] [--all]

If handoffs are disabled, any currently active handoffs in the specified direction will be terminated.

If the riak-admin "set" command is used to disable handoffs in a given direction via the newly exposed handoff.disable_inbound or handoff.disable_outbound settings, corresponding handoffs will also be automatically terminated.

This PR also includes some additional unit testing in riak_core_handoff_manager.
Remove the "usage:" at the beginning. it's already added by riak_cli. Although, it may be better to remove it from riak-cli in the future. not sure.