Update Phoenix generator to support Ecto+Mongo out of the box

[josevalim]

No description provided.

[michalmuskala]

I think we need to release a hex package first so there is no problem with versions.

[michalmuskala]

I was looking into this and it seems this will require quite a lot of changes to the phoenix.new generator:

• We need a different config/test.exs file (no Ecto.Adapters.SQL.Sandbox),
• change test helpers to use Mongo.Ecto.truncate/1 instead of Ecto.Adapters.SQL.restart_test_transaction/1
• change Web.model to override default @primary_key.
  There may be other things.