Metaclass evaluation

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

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There's a big potential of improvements regarding the metaclasses. We should definitely have some sort of evaluation, for code such as the following:

```
#!python

import six

class A(type):
def __call__(cls):
return 42

@six.add_metaclass(A)
class B(object):
pass

b = B()
print(b.f) # b is an integer

```

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/173

No source files or tests are named. Start by reproducing the `six.add_metaclass` example and tracing astroid's metaclass evaluation and inference behavior; clarify the intended result and scope before implementation. Done should include a defined evaluation behavior for the example and regression coverage.

Written by the indexing model from the issue text.