Use UnionType when having multiple return values

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

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This is partially related to #153.

Currently, in lot of places where we are expecting multiple values as an inference result, we either retrieve the first one, using next, or we are returning to the upstream caller an YES object, in case we have multiple values returned.
This is partially a problem, because it leads to a partial understanding of the code by astroid.
In this case, we should return an UnionType with all the possible values and we should operate on that UnionType result.

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/154

Start by tracing the inference paths that currently retrieve only the first value with next or return a YES object when multiple values are possible, and review the related issue #153. Done means those paths preserve all possible inferred values in a UnionType and operate on that result rather than losing alternatives.

Written by the indexing model from the issue text.