Understand version and platform checks

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

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(This is useful for flow control analysis)

Understand version and platform checks:

```
#!python

* if PY3 / PY2
* if sys.version_info..
* if os.name ==
* if platform.platform ==
```

These could be built-in in the flow inference.

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/113

Start by locating the flow-inference code and reviewing how conditional checks are currently represented. Compare that behavior with the listed PY2/PY3, sys.version_info, os.name, and platform.platform cases. Done should mean these version and platform checks are understood by flow inference for static analysis.

Written by the indexing model from the issue text.