add support for re-authentication

as discussed here, https://github.com/facebook/facebook-php-sdk-v4/pull/309

adds a new base method to generate url : makeUrl()
and other  3 methods to get a login URL, re-request URL & re-authentication URL. 
"Url" should be all-caps and sentence should end in period for consistency. :)

`Array of parameters to generate URL.`