SW-28 Allowed specifying multiple classes in @Map(of=...) parameter

Related issue: #49 

You can now specify multiple classes in `@Map(of=...)` parameter.
So you can write:
``` java
   @Map(to = "bar", of = {A.class, B.class})
   Bar bar;
```
instead of:
``` java
   @Mappings({
      @Map(to = "bar", of = A.class),
      @Map(to = "bar", of = B.class)
   })
   Bar bar;
```