use local squid docker instance for proxy testing

show more info on failed HTTP requests

hopefully this will help with figuring out why our HTTP tests sometimes
fail on travis.

run HTTP tests against EU server

this hopefully helps with recent problems of spradically failing tests.

fixed HTTPS proxy tests, our tests now run on httpbin.org

This also reverses the order of crypto protocols tried again. Using TLS
first again. related to #915

give better error message in HTTPClient for failed crypto setup

HTTPClient: correctly abort a proxy connection

if a needed CONNECT tunnel fails

more robust HTTP testing

connections are now retried after timeout and failing connections will
be marked as skipped instead of failing. This should reduce false alarms
on travis

make sure HTTP test uses HTTP only and doesn't get redirected

more verbosity in test case

changed proxy in test case

support CONNECT for tunneling SSL via HTTP proxies FS#2431

The included test cases currently expect a squid at localhost:3128