Extension for GPX Export (#157)

Added the possibility to to extend the GPX trackpoints by custom fields.

A discussion on different types of extensions can be found here:
https://groups.yahoo.com/neo/groups/gpsxml/conversations/topics/2049
However, a lot of the extensions listed here, don't fit into a file generated by GraphHopper - like temperature, or heartbeat frequency.

So I implemented some example extensions along the lines of the example given in the issue.
(http://osmz.ru/route_cloudmade.gpx).

Additional extensions should be easy to add. If users needs regarding these extensions are greatly varying, a external configuration might be useful to turn certain extensions on or off.