Import of turn restrictions (OSM).

OSM relations of type "restriction" will be imported. According to the
restriction type, several entries will be added to a turn cost table.
Furthermore, the turn flag encoder is now implemented by the current
edge encoders (e.g., foot flag encoder do not store any turn flags).
Import can be enabled by GraphHopper#setEnableTurnRestrictions(true),
which is disabled by default.