Added support for NSArray objects

for example

    Model *m1 = [[Model alloc] init];
    // ... set propertites for m1
    Model *m2 = [[Model alloc] init];
    // ... set propertites for m2

    NSArray * models = [NSArray arrayWithObjects:m1,m2, nil];

    NSData* jsonData = [models JSONData];
    NSLog([models JSONString]);