Prevent firing `page.onInitialized` event twice
Fix #337 ability to set headers only on the initial request

PhantomJS has `customHeaders` property but headers will be
sent to the server for every request. If we set Referer header
it will appear in every subsequent ajax request. PhantomJS doesn't
have any specific option to set headers just for the first request,
instead they suggest a workaround, you can reset headers inside
`onInitialized` callback. This commit adds new method
`driver.add_header(name, value, options)`, setting last parameter as
`permanent: false` you can set given headers only for the first request.