Update and unify dev scripts

[crbelaus]

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

Closes #111

[odarriba]

🚀