Show routes info and instructions with the preferred unit-system

[thanosbellos]

Upon successfull response, the routes info and instruction can now show
the distances with either metric or imperial units.We can do so by
setting an option on L.mapbox.directions().

-We can choose metric system setting option unit on directions creation
-Usage example :

    directions = L.mapbox.directions({units: 'metric'});

//if we do not set it or if we misspell it , imperial (miles) system will be used

-If we not specify units or if we misspell it, imperial system will be
used by default

[jfirebaugh]

Thanks! Committed as a99abf2.