Add basic undo/redo controls in the toolbar

[thibaudcolas]

Fixes #79.

[loicteixeira]

@thibaudcolas Shouldn't the whole ToolbarGroup be wrapped into the showUndoRedoControls check? Does it displays fine if the group is empty?

[thibaudcolas]

Yes, the ToolbarGroup contains some magic so it doesn't render anything when it has no children:

draftail/lib/components/ToolbarGroup.js

Lines 4 to 7 in 60f2f6e

	const ToolbarGroup = ({ children }) => {
	const hasChildren = React.Children.toArray(children).some(c => c !== null);
	return hasChildren ? <div className="toolbar-group">{children}</div> : null;
	};