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