Use Celery for asynchronous tasks.

Needs a running redis server - see INSTALL.md for short instructions.
celery_worker: Use config.to_envvar for command line config
worker: Added analyse_flight task
scripts: Added analyse_delayed.py
config: Renamed CELERY_BROKER_URL setting to BROKER_URL

see Celery documentation
views.upload: use async task for in-depth reanalysis of uploaded flights
views.flight: async reanalysis if possible
lib.xcsoar.analysis: reduce default triangle points

No longer needed, because the asynchronous analysis takes care of
the full analysis now.
lib.xcsoar.analysis: limit process time and memory
Merge pull request #82 from TobiasLohner/celery

Asynchronous tasks with Celery