Start phoenix router in the supervision tree of the generated app

Hey guys,

this PR contains a change I discussed with Chris over twitter. With this pull request the router will automatically started in and supervised by the supervisor of the generated phoenix app. This also means it's automatically started via iex -S mix and it doesn't need to specified via -e  in mix run anymore.

Oh, and I also fixed and unrelated test warning :-)

Hope you like it!

Cherio
Björn