Fix xul file address

When using  slimerJS with node-phantom, node-phantom will open a control page at <http://127.0.0.1:32884/>. Without this patch, next page open will open <http://127.0.0.1:32884/webpage.xul> instead of <chrome://slimerjs/content/webpage.xul>
Merge pull request #16 from vmeurisse/xulAddress

Fix xul file address