RUBY-1529 Adding ActiveSupport::TimeWithZone patch (#104)

* Adding ActiveSupport::TimeWithZone patch

* Decouple bson-ruby from activesupport

* Document ActiveSupport usage

* Undepend shared examples on AS

* Verify serialization correctness

* Test AS in evergreen

* Byebug is 2.0+ only

Author: zachmccormick

docs/tutorials/bson-v4.txt

@@ -34,7 +34,20 @@ To install the gem with bundler, include the following in your Gemfile:
 
     gem 'bson', '~> 4.0'
 
-The BSON gem is compatible with MRI >= 1.9.3, JRuby 1.7.x, and Rubinius 2.5.x
+The BSON gem is compatible with MRI >= 1.9.3, JRuby >= 9.1, and Rubinius 2.5.x.
+
+Use With ActiveSupport
+----------------------
+
+Serialization for ActiveSupport-defined classes, such as TimeWithZone, is
+not loaded by default to avoid a hard dependency of BSON on ActiveSupport.
+When using BSON in an application that also uses ActiveSupport, the
+ActiveSupport-related code must be explicitly required:
+
+.. code-block:: ruby
+
+    require 'bson'
+    require 'bson/active_support'
 
 BSON Serialization
 ------------------