Allow to generate floats using normal distribution

This allow us to generate points with more probability when we are
closer to a limit/key point.

The sum of 6 draws from a uniform distribution give as a draw of a
normal distribution centered in 3. This is commonly used in programming
for simplicity and it is proved by the central limit theorem.

Contributes to rantly-rb#29

Pair-programmed by @Ana06 and @vicgalle.

lib/rantly/generator.rb

@@ -141,8 +141,15 @@ def positive_integer
     range(0)
   end
 
-  def float
-    rand
+  def float(distribution=nil, center=0)
+    case distribution
+    when :normal
+      # Sum of 6 draws from a uniform distribution give as a draw of a normal
+      # distribution centered in 3 (central limit theorem).
+      [rand, rand, rand, rand, rand, rand].reduce(0, :+) - 3 + center
+    else
+      rand
+    end
   end
 
   def range(lo=nil,hi=nil)