Add BC test for #4851
Possible fix for isUnique backward compatibility
Merge pull request #4871 from rchavik/2.5-is-unique-test

2.5 Add isUnique test + possible fix
Make rule sniff less complicated and tidy up doc block.

Refs #4871
Merge branch 'master' into 2.6

Conflicts:
	lib/Cake/VERSION.txt
Merge branch '2.6' of github.com:cakephp/cakephp into 2.6