Added IHttpRouter interface

[lclarkmichalek]

I've been implementing my own router class recently, and I've felt the need for an interface for Http Routers in two ways:

• The UrlRouter public API is 90% convenience functions, with only 2 functions that actually do work (match and handleRequest). By using an interface, the convenience functions need not be explicitly implemented by any other routers, and new routers need only implement match and handleRequest.
• Having a base type for routers is really useful for creating generic/pluggable site components. I don't know what the standard way of organising vibe.d modules is, but I generally just expose 1 function to register routes. Currently it's not really possible to do this without specifying the type of the router, which isn't really great. Having a base type solves this problem.

This pull request adds a IHttpRouter interface that implements all of the convenience functions from UrlRouter and also modifies UrlRouter slightly to implement IHttpRouter (I changed the return value of match from void to IHttpRouter).

[s-ludwig]

Just out of curiosity, what does your router do different/additional?

In general I don't see any issue adding an interface, I'll just make some small inline notes that would be good to have handled before merging.

[s-ludwig]

Minor: would be good to have /** .. */ style comments for consistency

[lclarkmichalek]

I have 2 different routers. One which uses regex based parsing with named subgroups for parameter selection a la django (this relies on a patch to phobos that has yet to be merged) and another that allows the exposing of another router on a subpath i.e.

auto siterouter = new UrlRouter();
siterouter.get("/test", &testView);

auto forwardedrouter = new ForwardedRouter();
forwardedrouter.add_router("/subsite", siterouter);
forwardedrouter.get("/test", &testView2)
// So forwardedrouter will map GET /subsite/test -> testView, GET /test -> testView2

[s-ludwig]

Thanks, merging in...