add input element class to wrapper

Currently the wrapper is not compatible with bootstrap 3. Bootstrap 3 requires a class(form-control) on the input elements (input, checkbox, select, textarea).

This pull request adds inputElementClass(not sure if I am happy with this name) to the wrapper config. This class will be added to all of the input elements. 

All the easyForm inputs now require the wrapper config so I have moved that into a mixin.