Merge pull request #82 from TobiasLohner/celery

Asynchronous tasks with Celery
Merge pull request #90 from TobiasLohner/precision

Precision
tests: Added test_unit_formatter.py
lib.formatter.units: Fixed _format() for ndigits=0

"not ndigits" for ndigits=0 is also be evaluated as true and the setting
wouldn't have been taken into account.
lib.formatter.units: Simplified unit definitions

Merged common code
lib.formatter.units: Used namedtuple for clarity
lib.formatter.units: Added DEFAULT_*_UNIT constants
lib.formatter.units: Converted constants to uppercase
lib.formatter.units: Explicitly convert value to float

Needed for some uses of the Decimal column type
model.flight_phase: Use Float instead of Numeric

Numeric has far worse performance characteristics and the table size
decreases from 95 MB to 87 MB after this patch.
Merge pull request #92 from Turbo87/numeric

model.flight_phase: Use Float instead of Numeric
Merge pull request #91 from Turbo87/units

Units testing and simplification
Airspace: update german airspace