please add HtmlUnit

Hey @mnagel, we've been looking at possibly adding Selenium to handle this kind of thing. I like that HtmlUnit is headless, but it doesn't seem to have a good recorder like Selenium. I could be mistaken.

I also like the idea of UI-based JAR management.

We looked at Grape support a while back, but it posed some significant security and architectural issues. I'll talk to the collector dev team about this and see what we can do.