sort deps with --save

This will keep deps ordered in a nice way, from this:

```
"dependencies": {
    "koa": "~0.1.2",
    "koa-router": "~1.6.1",
    "co": "~3.0.1",
    "request": "~2.30.0",
    "colors": "~0.6.2",
    "async": "~0.2.9",
    "JSONStream": "~0.7.1",
    "qs": "~0.6.6",
    "event-stream": "~3.0.20",
    "json-array-stream": "~0.1.2",
    "koa-jsonp": "~0.0.3",
    "koa-mount": "~1.2.2",
    "koa-compress": "~1.0.0",
    "koa-favicon": "~1.0.1",
    "koa-response-time": "~1.0.1",
    "debug": "~0.7.4",
    "co-body": "0.0.1",
    "nano": "~4.2.1"
  },
```

to
```
"dependencies": {
  "async": "~0.2.9",
  "co": "~3.0.1",
  "colors": "~0.6.2",
  "co-body": "0.0.1",
  "debug": "~0.7.4",
  "json-array-stream": "~0.1.2",
  "JSONStream": "~0.7.1",
  "koa": "~0.1.2",
  "koa-jsonp": "~0.0.3",
  "koa-mount": "~1.2.2",
  "koa-router": "~1.6.1",
  "koa-favicon": "~1.0.1",
  "koa-compress": "~1.0.0",
  "koa-response-time": "~1.0.1",
  "nano": "~4.2.1",
  "request": "~2.30.0"
}
```

I think that a `--save --sort` or a `--save-sort` is necessary, but before go any further I wanted to check if this is something that could be potentially merged.

P.S. test are passing even though we need the test for this feature.