Added the possibility to set one additional value for both edges and
nodes. By default no additional values will be stored and current graphs
are still valid. Any implementation of ExtendedStorage can be put into
GraphHopperStorage, which take care of any additional storages, which
depends on nodes or edges ( e.g. turn cost tables) by reading/writing
those additional values. An example implementation can be found in
TurnCostStorage.

This will be mandandatory to implement support of turn restrictions
later.
Merge pull request #133 from khuebner/master

Preparing support of turn restrictions: Extended GraphStorage