Rewrote self-type tour

Author: travissarles

tutorials/tour/_posts/2017-02-13-explicitly-typed-self-references.md

@@ -0,0 +1,37 @@
+---
+layout: tutorial
+title: Self-type
+
+disqus: true
+
+tutorial: scala-tour
+categories: tour
+num: 25
+next-page: implicit-parameters
+previous-page: compound-types
+prerequisite-knowledge: nested-classes, mixin-class-composition
+---
+When writing extensible code, it can be useful to declare a self-type as a means of dependency injection.
+
+To use a self-type in a trait, write an identifier, the type of another trait to mix in, and a right fat arrow (e.g. `someIdentifier: SomeOtherTrait =>`).
+```tut
+trait User {
+  def username: String
+}
+
+trait Tweeter {
+	this: User =>  // reassign this
+	def tweet(tweetText: String) = println(s"$username: $tweetText")
+}
+
+class VerifiedTweeter(val username_ : String) extends Tweeter with User{
+	def username = s"real $username_"
+}
+
+val realBeyoncé = new VerifiedTweeter("Beyoncé")
+realBeyoncé.tweet("Just spilled my glass of lemonade")  // real Beyoncé: Just spilled my glass of lemonade
+```
+
+Because we said `this: User =>` in `trait Tweeter`, now the variable `username` is in scope for the `tweet` method. This also means that since `VerifiedTweeter` extends `Tweeter`, it must also mix-in `User` (using `with User`).
+
+Whenever a trait needs to mix-in another trait, a self-type can be used to bring the other trait's members into scope.