Use a solid library for handling responses to HTTP requests

When a request is sent to a tracker, the answer is currently read manually, with a code that is very likely to have failures.

We should replace this whole code by a lib that does the work for us, properly (whatever the reason why it hasn't been done before ;) ).

Feel free to modify this blueprint in order to add candidate libs.
The needed features are :
- can send GET requests with parameters
- can read HTTP responses' code and return the content and its length in a simple maneer.
- weak memory footprint
- no memory leaks