add modal-open class to modal container when open

Standard bootstrap modals add `modal-open` css class to document.body when they are opened to ensure proper overflow/scrolling of large modals.  This PR adds `modal-open` to the container of the react-bootstrap modal (which defaults to document.body unless otherwise specified).  This works as expected both for global modals and contained modals:
http://screencast.com/t/Uqhp66HD9Nlp

@pcardune @kellyrmilligan @mwaylonis @cmwelsh - You have all commented on or submitted possible solutions to this modal issue.  Can you verify that this PR satisfies your needs?