Merge branch '2.6' into 3.0

Conflicts:
	README.md
	VERSION.txt
	lib/Cake/Controller/Controller.php
	lib/Cake/Core/App.php
	lib/Cake/Model/Behavior/TranslateBehavior.php
	lib/Cake/Model/Datasource/CakeSession.php
	lib/Cake/Model/Datasource/DboSource.php
	lib/Cake/Model/Model.php
	lib/Cake/Test/Case/Model/Behavior/TranslateBehaviorTest.php
	lib/Cake/Test/Case/Model/Datasource/CakeSessionTest.php
	lib/Cake/Test/Case/Model/ModelReadTest.php
	lib/Cake/Test/Case/Model/ModelValidationTest.php
	lib/Cake/Test/Case/Model/ModelWriteTest.php
	lib/Cake/Test/Case/Utility/CakeTimeTest.php
	lib/Cake/Test/Case/View/Helper/TimeHelperTest.php
	lib/Cake/Utility/CakeTime.php
	lib/Cake/basics.php
	src/Filesystem/Folder.php
	src/Utility/Hash.php
	src/Validation/Validation.php
Allow float 0 to be considered empty.

Much like int 0, float 0 should also be 'empty'.

Refs #4924
Update Validation.php

Updated the docblock to make it more descriptive of what the rule values should be. As it's linked in the Book to the API docs.
Fix eg. code in docblock.
Consolidate doc block headings.