Custom inputs for boolean, check_boxes and radio_buttons. Also other layout fixes.

Patched input mappings in initializer so simpleform uses custom constructions for boolean, check_boxes and radio_buttons.

Fixes:

* Boolean inputs (as: :boolean)
* CollectionRadioButtons (as: :radio_buttons)
* CollectionCheckBoxes (as: check_boxes)
* Fix inline view for both collections, also fixed display of first item
  to have the propper margin-top.
* Added 2 columns width to labels to display pretty for horizontal form.
* Fixed input-preppend, input-append and add-on
* Fix error classes for inputs, its errors and the flash.
* Added some css to application.css so vertical form displays nicely.

I based my custom inputs on the code found here to override the mappings:
https://github.com/plataformatec/simple_form/blob/master/lib/simple_form/form_builder.rb
And the necessary methods to override from the inputs directory:
https://github.com/plataformatec/simple_form/tree/master/lib/simple_form/inputs

Please, let me know what you think and if it is helpful/mergeable :)