Add a test

test_known_relays_in_results(): search for relays, check if all good,
and store results in a list accessible from other test_*() cases for
later use (for get_statuses() checks, etc.)
Add running backend URL to README

Backend is currently running on a dev server; include URL in README.
Refactor NetStatusDocument API: write get_statuses

Split the network status document API point into get_statuses() and statuses()
This allows us to query the API from outside the request context, and so on;
hence we can query this part of the API from unit tests; and also prepare for
future modularization (so that torsearch could e.g. be used as a module in
another system, and so on.)