Understand mutation of tuples

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

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It would be nice if astroid infers the fact that the tuple has a new value here:

```
#!python

from astroid.builder import AstroidBuilder as ab
node = ab().string_build('''
x = (1, 2)
x+=(2,3)
''')
f = node['x']
print(f.infered()[0].as_string()) # should have 1, 2, 2, 3
```

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/88

Start with the reproducer using AstroidBuilder.string_build and node['x'].infered(), then trace how tuple augmented assignment is inferred. The work is done when the example reports the expected tuple value, (1, 2, 2, 3), with a focused regression test if an existing test location is identified.

Written by the indexing model from the issue text.