Project specific settings to overwrite user settings

My plan to bring jsbeautify to every project I work on is not always possible, so I need to disable some automatic behaviour then and again.

With this patch it is possible to have a project specific setting for the plugin that overrides the system-global setting. It affects the two current plugin settings "format_on_save" and "jsbeautifyrc_files".

Example sublime-project file:
```
{
    "folders": [{
        "path": "..."
    }],
    "settings": {
        "tab_size": 4,
        "JsFormat": {
            "format_on_save": false
        }
    }
}
```

* If JsFormat doesn't exist, all settings are taken from the user/default package settings
* If an entry in JsFormat doesn't exist, this setting is taken from the user/default package settings

In the example above, formatting will not happen on save for this project, even if globally enabled. 

Idea for implementation taken from [here](http://sublimetext.userecho.com/topic/118670-override-plugin-settings-per-project-via-sublime-project/)