Improve the way how .getattr works

Originally reported by: **Claudiu Popa (BitBucket: [PCManticore](http://bitbucket.org/PCManticore), GitHub: @PCManticore)**

---

There's a very good reference on how the dot access works, for classes, instances and metaclasses (http://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/).
It would be nice if Class.getattr would behave similarly, meaning that its return value should respect mro at least (it currently doesn't do this). This could fix a couple of false positives in pylint (https://bitbucket.org/logilab/pylint/issue/354/cannot-infer-constructor-arguments-with might be an example of this)

---
- Bitbucket: https://bitbucket.org/logilab/astroid/issue/93

Start with Class.getattr and compare its current behavior with the referenced description of Python dot access for classes, instances, and metaclasses. Verify how the return value currently handles method resolution order, then confirm that the behavior addresses the linked pylint false-positive example.

Written by the indexing model from the issue text.