Adding Description for Plugin Datafields

Pullrequest according to this feature request:
[http://www.impresspages.org/cms/answers/5954/adding-description-plugin-datafields-including-solution](http://www.impresspages.org/cms/answers/5954/adding-description-plugin-datafields-including-solution)

+
Some changes of css styles for input fields and an additional css class for the input type of password


Btw. Can you please add and link some labels for input types like the checkbox?