GPX export button in Graphhopperweb

This is the implementation for #113: GPX export button in Graphhopperweb. 
Additionally a minor bug was fixed such that the first configured vehicle is selected by default, and not "CAR", which does not work if e.g. only BIKE is configured.
Note: For some reasons (maybe DOS/LINUX line endings???) the diff of GraphHopperServlet.java and main.js shows every line as changed, although this is not case. I'm sorry for that, but I don't know where it comes from. In fact there are only a few changes in theses files.