Better dev tooling in app

**Don't merge this yet @brentonashworth :wink:, I wanted to submit this for your review**

- Use Piggieback library for better nREPL+CLJS support (i.e. it doesn't *not* work in Emacs - dumbrepl is no longer required)
- Include `io.pedestal.app-tools.dev` namespace instead of a locally
  generated dev.clj.
- Message after `(cljs-repl)` indicating where to visit for browser-connected REPL.

@brentonashworth I know you mentioned some issues you expected there to be with a non-local dev.clj, but in my experimentation I found file serving to occur correctly and pages to render properly.