Updated feature: Gleez Breadcrumb v2.0.0

* Renamed Breadcrumb::factory to Breadcrumb::instance
  because it is a singleton
* Removed @see link because repo not found
* Renamed all protected vars from $_var to $var — gradually move to PSR
* Added Breadcrumb::__toString, Breadcrumb::clear, Breadcrumb::getItems
* Used static::#instance instead self::$instance to enable inheritance
  and overriding
* Breadcrumb::addItem now return $this
Updated feature: Updated Breadcrumb Userguide
Merge pull request #716 from sergeyklay/feature-breadcrumb

Gleez Breadcrumb v2.0.0