Add a method to acquire tokens for mobile API

[vitche]

self.acquireToken = function() {
    var options = {
        host: 'oauth.vk.com',
        port: 443,
        path: '/access_token?grant_type=password&client_id=' + self.options.appID +
                '&client_secret=' + self.options.appSecret +
                '&grant_type=password&scope=notify,friends,photos,audio,video,docs,messages,notifications,offline,wall' +
                '&username=' + self.options.username +
                '&password=' + self.options.password
    };
    https.get(options, function(res) {
        var apiResponse = new String();
        res.setEncoding('utf8');
        res.on('data', function(chunk) {
            apiResponse += chunk;
        });
        res.on('end', function() {
            var o = JSON.parse(apiResponse);
            if (!o.access_token) {
                self.emit('acquireTokenNotReady', o);
            } else {
                self.token = o.access_token;
                self.emit('acquireTokenReady');
            }
        });
    });
};