Boilerplate for app that installs database fixtures

Hi,

This PR is meant to check if you're interested in incorporating these kind of extensions of active-slick.
I have no idea what vision you have for the project, so shoot it down if you don't want it.

This PR introduces a lightweight typeclass for classes that act as the data models, such that you can abstract over any of those classes easily, e.g.:

```Scala
  /**
   * Basic trait for database fixtures
   */
  abstract class Fixture[U : Model](val objects: Seq[U]) {
    val model = implicitly[Model[U]]

    def install()(implicit session: Session) = model.query.insertAll(objects : _*)
  }
```

This only works on classes that have a corresponsing `Model[U]` instance. 

Furthermore, it adds the necessary boilerplate for setting up a main class that allows you to (selectively) install database fixtures.

```Scala
object Fixture extends App
  with Fixtures
  with FixtureApp
  with UserFixture
  with BlogFixture {

  lazy val fixtures: Seq[Fixture[_]] = List(
    UserFixture,
    BlogFixture
  )

  main()
}

trait UserFixture { this: ActiveSlick with Auth with Fixtures =>
  object UserFixture extends Fixture(
    List(
      User(None, "some", "some@user.nl", "hashedandsaltedpassword", "salt!"),
    )
  )
}
```