[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
You can imagine that `<DefaultRoute handler={Foo} />` will be compiled into `{type: DefaultRoute, props: {handler: Foo}}`. I am pretty sure that RegisterRoute will just have to understand what DefaultRoute is.