Updates README requirements section
Huge update with lots of new changes:

- Moves constants and utilities to utils.py;
- Changes URL from '/route/.../' to '/.../';
- Validation of received parameters;
- Adds route_type (quickest/shortest) to URL;
- Adds parsing procedures for GPX track and route;
- Returns JSON dictionary with both cleaned GPX;
- Adds a server testing URL '/' with nice message.