[added] <DefaultRoute> component

Also, changed behavior of routes with no name, path, or children so
they act as default routes. <DefaultRoute> is essentially just sugar.

Fixes #164
Fixes #193
Do you by chance mean that something like `<Route to='somewhere' handler={ShellHandler} defaultHandler={DefaultHandler} />` would be more correct instead? Definitely easer then doing checks to make sure there is only one `DefaultRoute` child.