Implement riak-admin handoff enable/disable cmds

This implements the following commands:

riak-admin handoff <enable | disable> <inbound | outbound | both>
    [[--node | -n] <Node>] [--all]

It also adds cuttlefish config values for
handoff.disable_[inbound|outbound], and implements callbacks so that
disabling handoffs via the CLI will terminate any currently active
handoffs in the pertinent direction.

Finally, some unit tests have been enabled/added in the
riak_core_handoff_manager module to test the functionality that
terminates active handoffs.
Merge pull request #660 from basho/feature/riak-cli-handoff-enable

Adds CLI commands to enable/disable handoffs

Reviewed by @andrewjstone
Implementation of riak-admin handoff [summary|details] commands. In
addition to riak_core_handoff_status (the main module) the following
changes were made to other modules in support of this functionality.
* Refactored riak_core_vnode_manager:handle_call({repair...}) for readability.
* Refactor riak_core_vnode_manager:determine_handoff_target/4 into a series of function headers
* Create riak_core_vnode_manager:all_handoffs
* Some additional fixes for refactoring of handoff_type/ho_type into
  single cohesive type from 07e32b0