add "http://" scheme to ENV['http_proxy'] value if missing

This allows:

    http_proxy=localhost:8888 ruby myscript.rb

Omitting the scheme is already supported by curl, and this matches that
behavior.
don't swallow exceptions in em-synchrony
ensure consistent handling of connection failures

All adapters should now raise Faraday::Error::ConnectionFailed
handle connection failure in HTTPClient adapter
add HTTP proxy authentication support to EM adapters
add HTTP proxy support for Excon
fix HTTP proxy authentication for Patron
fix HTTP proxy authentication for net-http-persistent

Because net-http-persistent 2.8 doesn't unescape `user/password` values,
we override those accessors on the URI instance to return raw values.

References drbrain/net-http-persistent#48
consistently handle 407 Proxy Authentication Required for HTTPS requests
handle a special-case of connection failed with EM

https://travis-ci.org/lostisland/faraday/jobs/9489250
bump jruby-openssl to fix hang issues

It would hang indefinitely for HTTPS requests from JRuby 1.7.4 on my Mac
enable hash-style access to special Options properties

Makes these equivalent:

  options[:request] == options.request
  proxy[:password] == proxy.password
have script/server log to stdout when called directly
add integration tests for HTTP proxy support

Spins up a proxy server in a separate process, runs a few requests
through it with each adapter to verify that both proxy support and basic
authentication take effect.
teach proxy server to challenge unauthenticated requests

HTTPClient needs this challenge to retry with Basic authentication,
otherwise it won't attempt authentication at all.
hack HTTPClient to enable proxy integration tests

HTTPClient considers "localhost" to always be on a no-proxy list, thus
disabling our attempts to have integration tests on localhost. We
override its NO_PROXY_HOSTS constant during tests.
ignore Rubinius crashes

I cannot figure it out; I give up
bump multipart-post dependency for Ruby 2.0 compatibility
handle 407 Proxy Authentication Required with HTTPClient
Port fixes from v0.8.8 release