Allow to add the scale for the normal distribution

When generating floats using the normal distribution, allow to specify
the scale. This allow us to decide how close or far away should be the
generated points from the limit/key point.

Contributes to rantly-rb#29

Pair-programmed by @Ana06 and @vicgalle.

lib/rantly/generator.rb

@@ -141,12 +141,15 @@ def positive_integer
     range(0)
   end
 
-  def float(distribution=nil, center=0)
+  def float(distribution=nil, params={})
     case distribution
     when :normal
+      params[:center] ||= 0
+      params[:scale] || = 1
+      raise "The distribution scale should be greater than zero" unless params[:scale].positive?
       # Sum of 6 draws from a uniform distribution give as a draw of a normal
       # distribution centered in 3
-      [rand, rand, rand, rand, rand, rand].sum - 3 + center
+      ([rand, rand, rand, rand, rand, rand].sum - 3) * params[:scale] + center
     else
       rand
     end