Add support for userAgent

[Osmosis311]

https://github.com/ariya/phantomjs/blob/master/examples/useragent.js

[AlexanderYastrebov]

Vote up! For now one could use this hack:

function setUserAgent(page, ua) {
    page.setFn('EVIL_EVAL', 'page.settings.userAgent = "' + ua + '"');
}

I think API should be more broad to support setting of nested page properties like

page.set('settings.userAgent', ua);

but not only userAgent or any of page.settings