Add --output-format option to app-info command

E.g.:

    $ tsuru app-info --help
    Usage of :
    -a, --app (= "")
        The name of the app.
    --output-format (= "")
        Output to format (e.g.: "normal", "json", "prettyjson")

    $ tsuru app-info --output-format=json
    {"cname":[],"deploys":1,"ip":"tsuru-dashboard.10.128.40.70.xip.io",
     "name":"tsuru-dashboard","owner":"marca@mycompany.com",
     "plan":{"name":"autogenerated","memory":0,"swap":0,"cpushare":100},
     "platform":"python","ready":true,
     "repository":"git@tsuru.corp.mycompany.com:tsuru-dashboard.git",
     "teamowner":"admin","teams":["admin","devops"],
     "units":[{"Name":"d0a8595dd4ebbc15558e04fe33a74c9cc462e5551c03981d91a0006ce33f1962",
     "AppName":"tsuru-dashboard","Type":"python","Ip":"10.128.40.73",
     "Status":"started"}]}

    $ tsuru app-info --output-format=prettyjson
    {
        "Ip": "tsuru-dashboard.10.128.40.70.xip.io",
        "CName": [],
        "Name": "tsuru-dashboard",
        "Platform": "python",
        "Repository": "git@tsuru.corp.mycompany.com:tsuru-dashboard.git",
        "Teams": [
            "admin",
            "devops"
        ],
        "Units": [
            {
                "Name":
    "d0a8595dd4ebbc15558e04fe33a74c9cc462e5551c03981d91a0006ce33f1962",
                "Ip": "10.128.40.73",
                "Status": "started"
            }
        ],
        "Ready": true,
        "Owner": "marca@mycompany.com",
        "TeamOwner": "admin",
        "Deploys": 1
    }

Cc: @fsouza, @jblancett