Merge pull request #535 from gjaldon/static-path-helper

Endpoint.Adapter.static_path/2 with no cache

Some changes were applied on merge to guarantee we don't add
new asset files to the repository.
Do not cache static assets miss
cache_static_lookup should be false for default in dev
Use iodata in JSON too
Use phoenix_dep instead of changing mix.exs directly
Remove repeated HTTP config
Merge branch 'master' of https://github.com/phoenixframework/phoenix