Understand __missing__

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

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We could understand **missing**: when a key is not found in a dictionary, if it provides the dunder missing special method, then that method will be called and whatever is returned, will be the result of key access:

```
#!python

class A(dict):
def __missing__(self, key): return 42

a = A()
print(a[10])
```

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/145

Start by tracing how astroid handles dictionary key access and the __missing__ special method for dict subclasses, using the example in the issue as the expected behavior. Done means key access invokes __missing__ when the key is absent and uses its returned value.

Written by the indexing model from the issue text.