Support namedtuple's getitem

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

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We should detect namedtuple's indexing, as it is the case for this example:

```
#!python

from collections import namedtuple
f = namedtuple('a', 'a b')
c = f(1, 2)
print(c[0].lala)
```

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/182

Reproduce the supplied namedtuple example and trace how astroid infers c[0]. The issue names no files or tests, so first locate the namedtuple and getitem inference entry points, then add coverage showing that c[0].lala is understood as access to the namedtuple field.

Written by the indexing model from the issue text.