Use system proxies

Detected by checking the environment variables http_proxy, https_proxy, their
uppercase variants and then all_proxy. Uses no_proxy environment variable
as comma-separated list.

On Windows (and if previous detection fails), the registry is queried for
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings
This is what you edit when you open "Internet Options" > "Connections" from
the Control Panel or Internet Explorer
Add tests for proxy usage
Add tests for HttpProxy class
Add badges
Fix travis-CI link
Merge branch 'master' into feature/use-system-proxy
Make it possible to force direct connection (vs. using system proxy)

$c= new HttpConnection();
$c->setProxy(HttpProxy::NONE);
Remove superfluous "get" in getLastRequest() -> now simply lastRequest()
Allow specifying ports in excludes
Support `no_proxy=*`
See https://github.com/composer/composer/issues/1318#issuecomment-10328203
Support ".example.com" in excludes list
Support uppercase domains
Ugh:/
Support proxy excludes for URLs using IPv4 addresses
Add special-case handling for 127.0.0.1 / localhost
Support IP addresses in excludes
Extract exclude patterns to their own class; support netmasks
Test hosts are not added twice
Extract and test inferring system proxy settings
Honor environment on Windows and proceed with registry only if not existant
Create new package peer.http.proxy and move proxy-related package classes there