Make Baum more Postgres friendly.

- Override Eloquent\Model->newBaseQueryBuilder to return a custom Baum\Extensions\Query\Builder object.
- Implement an extension to Eloquent\Query\Builder which drops ORDER BY clause before executing an aggregate function.
- To accomplish the last point, implement a reOrderBy method which will replace the current scope ORDER BY clauses.
- Make tests Postgres friendly too.
Fix 'insideSubtree' method.
Add tests for impossible moves.