Change default package used for materialized classes to reflect Jackson 2.x

Currently (as of 2.5) mrbean still uses package `"org.codehaus.jackson.generated` for materialization.
But it should really use the new package:

```
com.fasterxml.jackson.module.mrbean.generated
```

both to avoid conflicts with Jackson 1.x mrbean (unlikely but possible), and to make it easier to see that they are related to Java class of mrbean itself.
