Improvements to proxy plugin

[treadie]

As you know, if there is a conflict with the CSS on a page, then the proxy panel will not be visible on the page. My current work around for sites that do not render the panel is to open arachni.proxy/panel in a new browser tab/window to control the functionality of the proxy.
It would be awesome if when the proxy plugin was being used, the WebUI would automatically load the arachni.proxy/panel/ into a frame on the WebUI scanning page. although maybe slightly less usable (rather than right on the page itself), it will be reliable and always there no matter the site/page being tested.