Add dispose support

[dlwr]

This pull request adds a dispose option to the gm instance.
The disposer consists of an EventEmitter and events. We can add multiple disposers.
e.g.

gm(dir + '/photo.JPG').options({
  disposers: [
    {
      emitter: emitter1,
      events: ['start']
    },
    {
      emitter: emitter2,
      events: ['end', 'error']
    }
  ]
});

or we can add after instanciation.
e.g

gm.addDisposer(emitter3, ['close', 'aborted']);

after this PR, for example, we can dispose immediately when node connection is aborted or closed.

before:

there are many convert processes after aborting requests.

after:

there are no convert processes left.

Best regards.

[dlwr]

this check was useless because child_process.connected is always false without forked process or not using ipc.
( please look arround here https://github.com/nodejs/node/blob/master/lib/internal/child_process.js#L337)
So, I change this line to check proc.exitCode at https://github.com/aheckmann/gm/pull/487/files#diff-b26080cc6f2472dce354dbedffc64d57R329

[aheckmann]

thanks!