Ensure enough param values in mount handler

Implement router tree Walker, an analogy to filepath.Walk (go-chi#222)

* Implement router tree Walker, an analogy to filepath.Walk

func Walk(r Routes, walkFn WalkFunc) error
type WalkFunc func(method string, route string, handler http.Handler, middlewares ...func(http.Handler) http.Handler) error

* Move Walker to tree.go

Flatten Context.URLParams data structure

Merge branch 'master' of github.com:go-chi/chi

v3.1.0 release

Update README with latest benchmarks of v3

Update README, CHANGELOG, other comments

Release v2.0.0 with support for go1.8, http2 and http.Pusher

Import ordering for travis

Ensure we dont route to an empty mux, fixes go-chi#42