Add HTTP input plugin + support for a proxy with http_post output plugin

1. Add HTTP input plugin for the server to be able to receive incoming logstash messages over HTTP/HTTPS. This is needed in situations where the agent resides in a location that is only able to access HTTP/HTTPS, e.g. behind a proxy.
2. Add support to the http_post output plugin to send data through a proxy (using the 'tunnel' module)
3. Make the http_post output plugin 'path' parameter optional with a default value of '/'