Added a 'syncselected' command to sync only selected files to the container

Added a management that can be used to sync only selected files from the local static media folder to the container. To tell which files to sync, it needs to be passed filenames or patterns (supports Unix shell style wildcards) as arguments. For eg,

```bash
$ django-admin.py syncselected "*.js" "*.css" 
```
will sync only the javascript and css files. 

Have also updated the docs. Let me know in case I should change something to match specific standards. 

Thanks