lib.xcsoar.*: use new xcsoar python module
views.flight: show 2 minutes before takeoff and after landing
lib.xcsoar_.analysis: add terrain elevation for takeoff/landing detection

Fixes #231
lib.xcsoar_.analysis: move flight times mangleing to separate function
lib.xcsoar_.analysis: select best free-flight period from flight

Include powered periods...
setup.py: add temporary dependency link