added tabs
add fixes
Merge branch 'upstream' into 365-bootstrap-tabs

* upstream: (122 commits)
  Refactored StringHelper.
  Line number tweak for error page
  added note about dependency to registerJsFile
  docs: Be explicitly clear about ActiveRecord::create()
  fixed AutoTimestamp example in advanced app
  \yii\rbac\DbManager::isAssigned() signature fix
  Changed the way to set timestamp for multiple attributes.
  Fixes issue #428 Allowing UniqueValidator to be used with non-AR models.
  Better exception page look.
  Removed unnecessary code from MSSQL schema.
  Hover and error line increased by 1 px.
  Error line number fix.
  Error line number fix.
  Error line number fix.
  Lift off!11 #418
  MSIE fix.
  Fixes (theoretically) #418 and #413.
  Removed echo
  RBAC DbManager fixes
  Always refetch the migration table definition.
  ...
Added dropdown widget
Tabs refactored
Enhance Dropdown to use items as  strings. Remove string=divider rule
Refactored based on comments and feedback (3rd round)
Refactored based on comments and feedback (3rd round)
silly typo
another typo fix
fix phpDoc
modified phpDoc
small fix
Merge pull request #392 from tonydspaniard/365-bootstrap-tabs

[ready]bootstrap tabs widget
refactoring button classes.
Merge branch 'master' of github.com:yiisoft/yii2
refactored ButtonDropdown and Dropdown.