Allow chaining in the addCrumb method

For convenience I think it might be valuable to allow addCrumb (and possibly other similar functions) to return $this to permit chained calls such as this:

    $this->Html->addCrumb('Admin', '/admin')->addCrumb('Blogs', '/admin/users')->addCrumb('Add');

as opposed to the considerably more verbose version that needs to be used now:

    $this->Html->addCrumb('Admin', '/admin');
    $this->Html->addCrumb('Blogs', '/admin/users');
    $this->Html->addCrumb('Add');

I'm not sure if this violates some API conventions for helpers but it does seem rather more convenient to work with to me.
Update HtmlHelper.php

Updated documentation to include the chained $this return
Merge pull request #3002 from moberemk/patch-2

Allow chaining in the addCrumb method