Use `entry` instead of deprecated `find_or_insert_with`.

Build against rust nightly `rustc 0.12.0-nightly (d64b4103d 2014-09-26 21:47:47 +0000)` fails due to 

```
src/router.rs:49:9: 50:64 error: use of deprecated item: use entry instead, #[deny(deprecated)] on by default
src/router.rs:49         self.routers
src/router.rs:50             .find_or_insert_with(method, |_| Recognizer::new())
error: aborting due to previous error
Could not compile `router`.
```

Use `entry` instead of `find_or_insert_with` to fix the error.