Python-leetchi

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- Added WithdrawalContribution resource

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- Signals handling via blinker
- Added **request_finished** signal to add a callback when a request is finished
- Added **request_started** signal to add a callback when a request is starting
- Added **pre_save** signal to add a callback when a new resource will be created
- Added **post_save** signal to add a callback when a new resource is created

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- Added **culture** parameter to Contribution to specify the language
used in payment.

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- Added time between the beginning and the end of the requests in logger.

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- Removed imports in `__init__.py`, use modules instead.

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- Added **type** attribute in Contribution resource to set the type of the
transacti n.