Starting to implement translations caching
Added the ability to merge the loaded translators from two different registries

This will become handy when implementing translator caching for real
Implemented cache for translator objects in I18n
Remocing code that at the end was not actually useful as it was growing
in complexity.
Improving docblocks and fixing test
Merge branch '3.0-i18n-next' of github.com:cakephp/cakephp into 3.0-i18n-next