Add Convert Functions

[leico]

usage:

ofxJSON json;
json["Rect"] = ofxJSON :: Convert( ofRectangle(100, 200, 300, 400) );

---

{
    "Rect"{
          "x"      : 100
        , "y"      : 200
        , "width"  : 300
        , "height" : 400
    }
}

if you like.