Fix Dropbox on Python 3.

- Manage a special case of error where we need to retry the request
Raise TimeoutError in tests Launcher
Fix support for stdout and stderr in tests Launcher
PEP8 fix
Escalator: Close the workers when the db is closed
Don't close sockets if they are not created yet
Plug: Don't delete the transfer key when aborting

This is not needed as the key will we rewritten when
the next transfer will start. Also it was causing issues
when the Onitu was killed but the driver had time to
delete the key, as the transfer was not restarted.
Add TestDriver.connect to get the current session
Add a new driver for the tests

This driver should be used instead of LocalStorage for
the tests.

It stores the files in the metadata, and get
notifications and requests via ZMQ.
Use the TestDriver in the tests
Fix tests for the API with the new test driver
Add test_driver to tox
Merge branch 'test_driver' into develop. Fix #98
Fix the token in get refresh token
Fix multiline string concatenation
Fix Referee's notifications with 3+ entries
Return the new metadata in Plug.move_file. Fix bug in LocalStorage.
Prevent KeyError in tests launcher
Move tests into tests/functional/core/
Fixtures