Child routers

Hi,

Great little library here, but I have an idea for an improvement - add support for nested routers. Currently the library is a little unsuitable for more complex URL hierarchies since all the paths must be defined at the same level. What I've done here is add support for child routers that can form a hierarchy, making parsing simple at the node level.

Not the tidiest implementation, perhaps, but it certainly works and test case included. Please let me know if you have any ideas for improving/tidying the implementation.

Thanks,

J