Use lein-ring for better dev server experience

- runs nrepl automatically
- opens page automatically when run with `lein ring server`
- maintains ability to execute with `lein run`; alternatively, can use
  `lein ring server-headless` for similar behavior.
- Fixes #6.
- Depends on #8.
Merge pull request #9 from mathias/lein-ring

Use lein-ring for better dev server experience