Implement system-level security and stress tests

[noexec]

• Properly test timeouts and retries
• Test multiple Download instances working in multiple threads
• Verify https is secure (algorithms, CAs)

[noexec]

Should also decide on default User-Agent (none, specific curl/version as it is now, libcurl/version from pycurl.version_info()[1], …)
(done in #13)

[noexec]

Ensure that file:// links are disallowed.
(done in #13)

[noexec]

Verify URL redirects policy:

• HTTP may redirect to HTTP and HTTPS
• HTTPS may only redirect to HTTPS

Consider implementing a more restricted URL redirects policy:

• Redirect should only work for same domain or subdomains, with same port (auth details should be stripped for comparison) — verification should be added via HEADERFUNCTION