Django-slowtests

- Return error code to shell in case of failed tests

- Added option to generate full tests reports by passing None to NUM_SLOW_TESTS

- Fixed cast error, leading to slow test ordering issues
- Fixed test case

- Handle django not installed case to ease installation

- Fixed report printed to console

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- Dropped Django 1.5* support
- Dropped Python 3.3* support
- Added Django 1.11.* support
- Fixed an issue preventing settings to be taken into account in some case (24)
- Added an option to generate a report on demand only using a command line
parameter
- Reports are now compatible with django tests' --parallel option
- Added an option to generate a json file containing the report, instead of
printing in to console.