Only access document.body after component has been mounted

document.body is null before the body tag has loaded, which means when
loading react-bootstrap.js in the `<head>` would result in the container
prop being null. This change moves the `document.body` accessing into
the `getDOMNode` fn which is only called after the component has been
mounted.

closes #180
Merge pull request #186 from pieterv/documentbody-accessing

Only access document.body after component has been mounted