Output global bbox to JSON

First attempt to address issue #74.

 **There's one thing which deserves closer attention**: Printing out the initial "elements" tag moved from write_json_header to print_elements_header in frontend/print_target.cc.

I did a number of tests with osm-script/ql and json and they looked good so far. But I'm not 100% sure if there's some use case where write_json_header is called, but not the new method print_elements_header. This would result in invalid json, as Web_Output::write_footer would assume an opening "elements" tag in any case, which is then missing.

In any case this patch might need some rework... feedback welcome!

Fixes #74