Adds :selector/:adjustment to COMPLEX_ATTRIBUTES for Group, Leaderboard, and Definition.

@suprajanarasimhan your [previous commit](https://github.com/badgeville/badgeville-ruby/commit/d609b94fc9b1f51e6e8bd0228613ee933a3eed90) did in fact fix the `custom` attribute on `Player` but because you removed the check for:

```
if [:selector, :adjustment].include?(key)
```

in `BaseResource`.  You need to set `COMPLEX_ATTRIBUTES` in `Group`, `Leaderboard`, and `Definition` or the complex attributes in those classes break.

The other place in the API I see a `COMPLEX_ATTRIBUTE` in the API is in Level but it doesn't appear this gem supports `Level`.