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
Fix yoda condition in File test
Merge pull request #3186 from bcrowe/hotfix-yoda-test

Fix yoda condition in File test
Adds phpunit required version to TestShell "not found" error message.
Merge pull request #3193 from beporter/feature/phpunit-required-version

Adds phpunit required version to TestShell "not found" error message.
cs
more missing doc block tags added
more cs
fix indentation
correct missed cs errors
Merge pull request #3197 from dereuromark/master-cs

cs
Change argument type to mixed in docblock, as debug() and pr() accept not just a single type
adding options for react to response when asset not exists
correct deprecation notices
generalize sentence
Merge pull request #3224 from dereuromark/master-deprecations

correct deprecation notices
Only count `_id` if it is the last 3 characters in the field name.

This prevents issues when baking models for tables containing `_id`.

Fixes #3230
Fix email rendering when using 2 different plugins.

When an email template and layout are in different plugins the incorrect
plugin would be used for the layout.

Fixes #3062
Add file missed in last commit.