Fixed typo in upgrade guide

Fixes #1535: Improved `yii\web\User` to start session only when needed. Also prepared it for use without session.

http://docs.travis-ci.com/user/build-configuration/#Rows-That-are-Allowed-To-Fail

fix doc code according to yii2 core code style

in this version a bug that affects markdown display has been fixed.

fixes #2589

fix doc code according to yii2 core code style (When instantiating class it should be new MyClass(); instead of new MyClass;.)

Fixes #1564: Removed `yii\web\Session::autoStart` and added `hasSessionId`. Session will be automatically started when accessing session data

for prev, next, first and last page.
http://www.w3.org/TR/html401/struct/links.html#h-12.1.2

SELECT DISTINCT queries made subqueries in Query::queryScalar 

Use the current appliaction language in DatePicker

Changed `Controller::afterAction()`, `Module::afterAction()` and `ActionFilter::afterAction()` to pass `$result` by value instead of reference

added server name, for creating url rules

Change AssetBundle.register() return type to static

Enable IDE typehints for custom AssetBundle properties (related to #2572).