Feature/json lib interface

- [x] make output generation in `Plugins/*.h` format agnostic
- [ ] implement output format specific code in calling code:
 - [x] RequestHandler
 - [x] Tools/simpleclient.cpp
 - [ ] Node bindings
 - [x] add xml/gpx rendering for JSON Container
 - [x] replace hand-rolled GPX generator

supersedes #1261