Remove DateTime implementation, remove Delegation

DateTime inherits from Date, so we don't need to implement in both. And
the DateTime implementation was wrong, anyway.

Using SimpleDelegation for Gigaseconds was a bad idea as delegation
wasn't needed. Imagine this code:

```ruby
d = Gigasecond.new(Date.today)
irb(main):011:0> d.date
irb(main):012:0> d.to_time
```

`date` returns a date 32 years in the future, while `to_time` returns
today. Bad idea.

ruby/gigasecond/gigasecond.rb

@@ -1,14 +1,16 @@
 require 'delegate'
-class Gigasecond < SimpleDelegator
-  def date
-    __getobj__.gigaseconds_since(1)
+class Gigasecond
+  def initialize(start_date)
+    self.start_date = start_date
   end
-end
 
-class DateTime
-  def gigaseconds_since(multiple)
-    self + multiple.gigaseconds
+  def date
+    start_date.gigaseconds_since(1)
   end
+
+  private
+
+  attr_accessor :start_date
 end
 
 class Date

ruby/gigasecond/gigasecond_test.rb

@@ -26,7 +26,7 @@ def test_datetime
     1000.times do |x|
       random_datetime = Time.at(rand * Time.now.to_i).to_datetime
       random_gigaseconds = rand(1000)
-      expected = random_datetime + (10**9 * random_gigaseconds)
+      expected = random_datetime + (10**9 * random_gigaseconds / (24 * 60 * 60))
 
       assert_equal expected, random_datetime.gigaseconds_since(random_gigaseconds), "DateTime: #{random_datetime} plus #{random_gigaseconds} gigaseconds should be #{expected} using gigaseconds_since"
     end