Allow inbound transformers for property with specific classes.

**I have class BGRoom with property**

`@property (strong, nonatomic) BGChannel *channel;`

And my JSON looks like this
`{
    ...
    "roomChannel": "42"
    ...
}`

I write JSON mapping 
`return @{
             ....
             @"roomChannel": @"channel",
             ....
             };`

I want to write transformer for channel property like this
```
@implementation StringChannelTransformer

- (id)transformedValue:(NSString*)value {
    if (!value) return nil;
    return [BGChannel objectForPrimaryKey:value];
}
```