Delaying next() callback until after all uploads are complete

This should fix the issues discussed in https://github.com/bobobo1618/docpad-plugin-sunny/issues/10 and https://github.com/bobobo1618/docpad-plugin-sunny/issues/11

I did not change the version number. I would recommend that you `npm unpublish docpad-plugin-sunny@2.0.13` and then bump the version to 3.0.0, or at least 2.1.0 and `npm publish` that since we're completely changing the commands for using this plugin. 

I also hit on instance of this when testing it:

    (node) warning: possible EventEmitter memory leak detected. 11 listeners added. Use emitter.setMaxListeners() to increase limit.
    Trace
        at EventEmitter.addListener (events.js:160:15)
        at new WriteStream (/Users/nathan/docpad-plugin-sunny/node_modules/sunny/lib/stream.js:423:19)
        at AwsBlob.put (/Users/nathan/docpad-plugin-sunny/node_modules/sunny/lib/provider/aws/blob/blob.js:135:12)
        at Container.putBlob (/Users/nathan/docpad-plugin-sunny/node_modules/sunny/lib/base/blob/container.js:234:35)
        at uploadData (/Users/nathan/docpad-plugin-sunny/sunny.plugin.js:27:31)
        at Task.method (/Users/nathan/docpad-plugin-sunny/sunny.plugin.js:90:16)
        at ambi (/Users/nathan/docpad-plugin-sunny/node_modules/taskgroup/node_modules/ambi/out/lib/ambi.js:23:18)
        at /Users/nathan/docpad-plugin-sunny/node_modules/taskgroup/out/lib/taskgroup.js:124:23
        at b (domain.js:183:18)
        at Domain.run (domain.js:123:23)
        at Task.fire (/Users/nathan/docpad-plugin-sunny/node_modules/taskgroup/out/lib/taskgroup.js:121:23)
        at process._tickDomainCallback (node.js:459:13)

However, I think that's an issue in sunny, and regardless, everything seemed to work. So I'm leaving it for now.