Fix getattr compatibility issues and add getattr tests

[fjxmlzn]

The library gives incorrect behavior with getattr:

>>> from addict import Dict
>>> body = Dict(a=1)
>>> body.freeze()
>>> getattr(body, 'missing', 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../addict/addict.py", line 67, in __getattr__
    return self.__getitem__(item)
  File ".../addict/addict.py", line 71, in __missing__
    raise KeyError(name)
KeyError: 'missing'

The expected result should be 2.

This pull request fixes the issue, and adds related getattr tests.

[fjxmlzn]

This pull request is replaced by a better implementation #146