GtfsLibrary: Sypport HGV route_types extension

Support all HGV route_types. Some already supported by Google Transit and also in some official feeds (Berlin OpenVBB)

Source of the list: https://groups.google.com/forum/?fromgroups=#!msg/gtfs-changes/keT5rTPS7Y0/71uMz2l6ke0J
API: Add GTFS routetype to leg.

In a feed such as the Berlin feed it's very useful in order to distinguish between S-Bahn, U-Bahn, RE bahn etc.
API: fill in routeType on leg
Merge pull request #955 from skywave/routetypeextension

Add support for the HGV route_types extensions.