feat(modal): Toggle 'modal-open' class on body tag, as vanilla bootstrap JS does

[peteorpeter]

The 'modal-open' class is used to disable scrolling on the body element, which is key to getting the user's flow in tact when they close the modal window. If scrolling is allowed behind the modal, it is easy to accidentally scroll the background and lose your place when you return to the page.

This is the default behavior in the vanilla bootstrap JS (at least as of 2.3.2). It works nicely as a reasonable default behavior - to disable it, a developer can just comment out the corresponding CSS rule. In the angular-ui world, it might be nicer to add a configuration option for disabling the behavior.