update the document, and use Cu.reportError replace the old logs
To fix issue #28, I set browser.newtab.url if 'load in blank tab' is enabled, however, I think it is not a good idea since if SuperStart set browser.newtab.url, it won't get a chance to set it back if it is disabled or uninstalled. So I add a new flag 'extensions.superstart.set.browser.newtab.url', only both it and 'extension.superstart.load.in.blanktab' are true, SuperStart set 'browser.newtab.url' to 'about:superstart'. I think this way should be better. And thus I changed the version to 6.12.1