Add HTTP proxy support incl. authentication if necessary

JVM parameters for host, port, user, password are supported and can be
specified on the command line e.g. like this (without line feeds):

```
java
  -Dhttp.proxyHost=localhost -Dhttp.proxyPort=8080
  -Dhttp.proxyUser=kriegaex -Dhttp.proxyPassword="test äöüX"
```