Add base classes (or mixins) for the type of nodes

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

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We should add a couple of base classes (or mixins) in astroid for exporting the type of capabilites a node can have. For instance, Callable needs to implement `infer_call_result`, Iterable needs to implement `itered`. Sequence needs to implement `getitem` and so on. This would be extremely helpful when adding new nodes, especially when adding new objects such as objects.Super or objects.FrozenSet.

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/133

Start by reviewing astroid's existing node classes and how the Callable, Iterable, and Sequence capabilities are currently represented. Trace infer_call_result, itered, and getitem, then compare the needs of objects.Super and objects.FrozenSet; done means the capability design is agreed and supports adding these node types.

Written by the indexing model from the issue text.