Rewrote by-name parameters section of tour

tutorials/tour/_posts/2017-02-13-by-name-parameters.md

@@ -0,0 +1,40 @@
+---
+layout: tutorial
+title: By-name Parameters
+
+disqus: true
+
+tutorial: scala-tour
+categories: tour
+num: 31
+next-page: annotations
+previous-page: operators
+---
+
+_By-name parameters_ are only evaluated when used. They are in contrast to _by-value parameters_. To make a parameter called by-name, simply prepend `=>` to its type.
+```tut
+def calculate(input: => Int) = input * 37
+```
+By-name parameters have the the advantage that they are not evaluated if they aren't used in the function body. On the other hand, by-value parameters have the advantage that they are evaluated only once.
+
+Here's an example of how we could implement a while loop:
+
+```tut
+def whileLoop(condition: => Boolean)(body: => Unit): Unit =
+  if (condition) {
+    body
+    whileLoop(condition)(body)
+  }
+
+var i = 2
+
+whileLoop (i > 0) {
+  println(i)
+  i -= 1
+}  // prints 2 1
+```
+The method `whileLoop` uses multiple parameter lists to take a condition and a body of the loop. If the `condition` is true, the `body` is executed and then a recursive call to whileLoop is made. If the `condition` is false, the body is never evaluated because we prepended `=>` to the type of `body`.
+
+Now when we pass `i > 0` as our `condition` and `println(i); i-= 1` as the `body`, it behaves like the standard while loop in many languages. 
+
+This ability to delay evaluation of a parameter until it is used can help performance if the parameter is computationally intensive to evaluate or a longer-running block of code such as fetching a URL.