B3j0f-aop

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- fix sharing of exec_ctx between multi joinpoint execution instead of one per execution.

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- remove depency to future.
- update file hierarchy in order to leave the class Advice from the advice.core module.
- add requirements.txt file.

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- add support for pypy.

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- fix wrong import of builtins in python2.7.
- add tox file for a better multi-test environment.

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- add dependency to six and future.
- add support for python3.5.

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- add reference to advice and joinpoint members in the main package.