Django-mercadopago

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* Improve the documentation.
* Move repository got GitHub + Travis CI.

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* Fix crash introduced in v5.0.0

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* Drop compatibility with pre-4.3.0 callback URLs.
* There are now separate endpoints for successful/failed/pending payments. Each
one will then redirect to a separate URL, which must now be defined in the
configuration (see README for details).
* Settings are now contained in a single ``dict``, which should keep your
Django settings file tidier. Its entries can also be modified at runtime, if
needed.

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* Show a currently valid access token in the Account admin.

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* Support yet another notification format (apparently only used for merchant
orders though).

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* Fix notifications view crashing when receiving a POST for a merchant order
(it seems these notifications are messed up and have rubbish).
* Notifications endpoints now return JSON (which should make testing apps
slightly more pleasant).