Cast error in RegularLevel and Heap

Hi !

First of all, thanks for releasing the source code of your amazing game !

Unfortunately, the code does not compile as is on my station (java 6). It fails with two similar casting errors, in Heap.java and RegularLevel.java. I looked it up, and the statement
 
 (Collection<? extends Item>) bundle.getCollection(ITEM)

seems bogus to me: Item implements Bundlable, not the opposite. The error is very similar in RegularLevel.java.

I tried to fix it but I'm not a Java Guru, so feel free to reject my pull request and provide a better version of this fix. :]