Merge pull request #207 from Turbo87/i18n

Updated translations and template
forms/select: Renamed classes to avoid name clashing
forms/pilot: Use GroupSelectField for ClubPilotsSelectField

This results e.g. in the following content:

[unspecified]
Tobias Bieniek
LV Aachen (group)
  + Pilot A
  + Pilot B
  + Pilot C
forms/validators: Added NotEqualTo validator class

This is a copy of the EqualTo validator from wtforms, but negated.
forms/flight: Check that pilot and co-pilot are not the same
Merge pull request #211 from Turbo87/pilot-selection

Improved Pilot Selection
form/validators: Replaced NotEqualTo validator by CompareTo

We need to allow the user to set both pilot and copilot to "unspecified"
model/user: Generate tracking key in User() constructor
scripts/tracking: Added fill_missing_keys.py
model/user: Set tracking_key column not nullable
model/flight: Added pilot_name and co_pilot_name columns
templates: Show pilot name fallbacks if necessary
forms: Allow setting fallback pilot names
views/about: Use markdown format for imprint output
config: Added utf-8 header
Merge pull request #212 from Turbo87/unregistered-pilots

Allow unregistered pilot names for flights
views: Added error handler
views/api: Improved JSONP wrapper
views/api: Added error handlers for LookupError, TypeError and ValueError
views/api: Moved airport API logic into separate api package