Support for parsing a collection of collections

Something that could be done with a loop:

``` objc
NSArray *objs = ...
NSMutableArray *decoded = [NSMutableArray new];
for (NSDictionary *obj in objs) {
[decoded addObject:[SomeObject objectWith...]];
}
```

Can be simplified to something like:

``` objc
NSArray *objs = ...
NSArray *decoded = [SomeObject objectsWith...: objs];
```