Add and remove class "modal-open" to/from body

When the modal is too long it adds an additional scrollbar.
Bootstrap fixes this by adding 'modal-open' to the body.
We can do this too!
http://getbootstrap.com/javascript/#modals-usage
Merge pull request #132 from patricklx/patch-2

Add and remove class "modal-open" to/from body
apply my fork