Makes designated HOV way's routability configurable in profiles.

Why only `hov=designated` and not all access tags, such as `hov:yes`,
`hov=no` and so on? From the Wiki:

- designated: The way is designated to high occupancy vehicles.
- yes: High occupancy vehicles are allowed. This by itself does not imply that other vehicles are restricted from using the way.
- no: High occupancy vehicles are not allowed on the way. This by itself does not imply that other vehicle types are allowed to use it.

The primary use-case is conditionally filtering ways such as:

http://www.openstreetmap.org/way/11198593#map=19/37.82571/-122.30521&layers=D

In addition there is a notion of HOV lanes for lane handling:

http://wiki.openstreetmap.org/wiki/Key:hov#hov:lanes.3D.2A

This changeset does not handle lanes at all, only designated HOV ways.
For HOV lane support, a logic similar to the lane access handling needs
to be implemented. This needs to go hand in hand with the existing lane
handling introduced in:

https://github.com/Project-OSRM/osrm-backend/commit/7d076e93445a4d016422a575f1132b267cd13c29

References:
- #2711
- http://wiki.openstreetmap.org/wiki/Key:access
- http://wiki.openstreetmap.org/wiki/Key:hov#Values

taginfo.json

@@ -46,6 +46,12 @@
             "object_types": [ "way" ],
             "description": "Roads with area=yes are ignored by default."
         },
+        {
+            "key": "hov",
+            "value": "designated",
+            "object_types": [ "way" ],
+            "description": "Roads with hov=designated are ignored by default."
+        },
         {
             "key": "impassable",
             "description": "This is used by HOT."