Adding callback_url support to OAuth

Hi,

Currently Jaggery OAuth module doesn't support OAuth callback urls. After this Jaggery developer can send callback url in provider as follows:

var provider = {
	"oauth_version"     : "1",
	"authorization_url" : "https://api.twitter.com/oauth/authorize",
	"access_token_url"  : "https://api.twitter.com/oauth/access_token",
	"request_token_url" : "https://api.twitter.com/oauth/request_token",
	"api_key"           : "API_KEY",
	"api_secret"        : "API_SECRET",
	"callback_url"	: "http://127.0.0.1:9763/test/"
};

Thanks!