Add margin to checkbox and radio inputs.

With inputs nested in labels we'll need a bit of margin to space out
elements.

Refs cakephp/cakephp#4929
Merge pull request #166 from cakephp/nested-input

Add margin to checkbox and radio inputs.
Update exception class name.
Merge pull request #167 from cakephp/template-exception

Update exception class name.
Merge branch 'master' of https://github.com/cakephp/app