Update and unify dev scripts (#119)

This pull request replaces the previous `live.exs` and `dev.exs` scripts
with a new `dev.exs` script based on the Phoenix Playground project
that includes both the Controller and LiveView examples.

There are multiple benefits of this refactor:
- We can work using a single script instead of two
- Phoenix Plaground provides autoreload functionality by default
- Phoenix Playground simplifies process management and can start a
supervision tree
- Dependencies are automatically fetched and compiled separately from
those in the `mix.exs` file.

To start the dev server you just run:

```
iex dev.exs
```

README.md

@@ -62,10 +62,8 @@ mix assets.watch
 
 ### Development server
 
-We have a `dev.exs` script that starts a development server.
-
-To run it together with an `IEx` console you can do:
+We have a `dev.exs` script based on [Phoenix Playground](https://github.com/phoenix-playground/phoenix_playground) that starts a development server.
 
 ```
-iex -S mix dev
+iex dev.exs
 ```