getId() -
Method in class play.modules.morphia.Model
MorphiaEnhancer will override this method for sub class without \@Embedded
annotation specified
If user defined customized \@Id field, it's better to override this
method for the sake of performance.
Helper class to help serialize java object like BitSet
The code dealing with BitSet/byte[] conversion comes from
http://stackoverflow.com/questions/1378171/writing-a-bitset-to-a-file-in-java