Improve astroid's inference for simplejson & json

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

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The following stackoverflow post http://stackoverflow.com/questions/21545409/pylint-reports-maybe-no-member-error-for-data-loaded-via-simplejson describes a false positive
related to simplejson and json. It's more of an inference issue, so this should be fixed in astroid, rather than pylint.

```
#!python

import astroid.builder
builder = astroid.builder.AstroidBuilder()
ast = builder.string_build("""
import simplejson
import json
x = json.loads('"test"')
y = simplejson.loads('"test"')
""")

json_assignment, simplejson_assignment = list(ast.get_children())[2:]
print("json:", list(json_assignment.get_children())[-1].infered())
print("simplejson:", list(simplejson_assignment.get_children())[-1].infered())
```

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- Bitbucket: https://bitbucket.org/logilab/astroid/issue/75

Begin with astroid.builder.AstroidBuilder.string_build and inspect the inferred nodes produced for json.loads and simplejson.loads. Use the supplied reproduction to compare both results; done means inference no longer produces the reported false positive for the two equivalent calls. No test file is named in the issue, so locate the relevant inference tests before changing behavior.

Written by the indexing model from the issue text.