Cleanup roaring integration code

Mainly fix code style, rename variables, clean up some logic
@xvrl 

MutableRoaringBitmap.toImmutableRoaringBitmap was available up until 0.4.1 and it was buggy. It is no longer part of Roaring as of version 0.4.2.

So I am not sure what you mean. If you tell me that toImmutableRoaringBitmap (as in Roaring 0.4.1) gives you garbage, then yes, it very well might. I am sorry about this. The code was definitively buggy and untested. I recommend that it be never used. It slipped by me.

Or do you mean that the following code is buggy...

    MutableRoaringBitmap r = ...
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    DataOutputStream dos = new DataOutputStream(bos);
    r.serialize(dos);
    dos.close();
    ByteBuffer bb = ByteBuffer.wrap(bos.toByteArray());
    ImmutableRoaringBitmap ir = new ImmutableRoaringBitmap(bb);

Does this code work? It should.

By work, I mean that ir.equals(r) should be true at the end of the procedure.