Support for configuration for visible columns

Hi,

like discussed in last pull request, I implemented configuration for visible columns, label definition and enable/disable loading of parent.
Here is an example, how the configuration can be done. 
If the configuration is not available, then meta fields and source fields are displayed like before.

I tested it with in my application and it works fine.

{
    "fileInfo" : {
        "_parent"           :{"type" : "container"},
        "name"              : {"type" : "string"},
        "date"                : {"type" : "date"},
        "fileType"	         : {"type" : "string", "include_in_all" : false},
        "size"		 : {"type" : "long"},
        "_meta" : {
            "visualConfig" : {
            	"loadParents"  	: true,
            	"browser" 		: {
                	"columns" 		: {
                		"_source.name" 			: {"label":"Name"},
                		"_source.fileType"			: {"label":"Type"},
                		"_source.date"				: {"label":"Date"},
                		"_source.size"				: {"label":"Size"},
                		"_parent._source.folder"	        : {"label":"Folder"}
                	}
                }
            }
        }
    }
}