Add possibility to specify proxy to be used by HtmlUnitDriver

closes #80

From now on user can specify proxy for HtmlUnitDriver so &lt;preloadSources&gt; won't fail build in such a case when URLs has been specified in &lt;source&gt; element instead of relative local paths.

Example:

```
mvn test -Dhttp.proxyHost=myproxyhost.com -Dhttp.proxyPort=8000
```