8.0 project_gtd

The `project_gtd` module has been removed from Odoo 8.0, and this PR aims to integrate this module to OCA.
The last version of the source code of the module has been retrieved from the 8.0 branch, then fixed/cleaned/improved a bit.
Translations are not updated yet (I don't know if there is a tool to manage that since the Github migration).

It seems that the build fails against the `service_desk_issue` module.