Turn off loading the web page, or provide an extension to delay it until task is done

BrowserSupportBasedHyperlink loads the web page just after the server starts.

When loading a gwt application after the code server starts it pops up a javascript alert, locking the ui. This is because the code server starts after the app engine server and builds the nocache.js. It takes ~5 seconds.

Proposed solution is either turn it off, and let the developer initiate it after code server has finished compiling or provide an extension where gwt eclipse plugin could ask for it to open after it's ready.

(issue result of comments here: https://github.com/GoogleCloudPlatform/google-cloud-eclipse/pull/918#issuecomment-257440349)