Convert CHANGELOGs to Markdown format.

Reasons:

 * Markdown reads well as plain text, but can also be formatted.
 * It will make it easier for people to read on the web as Github
   formats the Markdown nicely.
 * It will encourage a level of consistency when people are writing
   CHANGELOG entries.

The script used to perform the conversion is at
https://gist.github.com/1339263

activemodel/CHANGELOG.md

@@ -0,0 +1,109 @@
+*   Added ActiveModel::Errors#added? to check if a specific error has been added *Martin Svalin*
+
+*   Add ability to define strict validation(with :strict => true option) that always raises exception when fails *Bogdan Gusiev*
+
+*   Deprecate "Model.model_name.partial_path" in favor of "model.to_partial_path" *Grant Hutchins, Peter Jaros*
+
+*   Provide mass_assignment_sanitizer as an easy API to replace the sanitizer behavior. Also support both :logger (default) and :strict sanitizer behavior *Bogdan Gusiev*
+
+## Rails 3.1.0 (August 30, 2011) ##
+
+*   Alternate I18n namespace lookup is no longer supported.
+    Instead of "activerecord.models.admins.post", do "activerecord.models.admins/post" instead *José Valim*
+
+*   attr_accessible and friends now accepts :as as option to specify a role *Josh Kalderimis*
+
+*   Add support for proc or lambda as an option for InclusionValidator,
+    ExclusionValidator, and FormatValidator *Prem Sichanugrist*
+
+    You can now supply Proc, lambda, or anything that respond to #call in those
+    validations, and it will be called with current record as an argument.
+    That given proc or lambda must returns an object which respond to #include? for
+    InclusionValidator and ExclusionValidator, and returns a regular expression
+    object for FormatValidator.
+
+*   Added ActiveModel::SecurePassword to encapsulate dead-simple password usage with BCrypt encryption and salting *DHH*
+
+*   ActiveModel::AttributeMethods allows attributes to be defined on demand *Alexander Uvarov*
+
+*   Add support for selectively enabling/disabling observers *Myron Marston*
+
+
+## Rails 3.0.7 (April 18, 2011) ##
+
+*   No changes.
+
+
+*   Rails 3.0.6 (April 5, 2011)
+
+*   Fix when database column name has some symbolic characters (e.g. Oracle CASE# VARCHAR2(20)) #5818 #6850 *Robert Pankowecki, Santiago Pastorino*
+
+*   Fix length validation for fixnums #6556 *Andriy Tyurnikov*
+
+*   Fix i18n key collision with namespaced models #6448 *yves.senn*
+
+
+## Rails 3.0.5 (February 26, 2011) ##
+
+*   No changes.
+
+
+## Rails 3.0.4 (February 8, 2011) ##
+
+*   No changes.
+
+
+## Rails 3.0.3 (November 16, 2010) ##
+
+*   No changes.
+
+
+## Rails 3.0.2 (November 15, 2010) ##
+
+*   No changes
+
+
+## Rails 3.0.1 (October 15, 2010) ##
+
+*   No Changes, just a version bump.
+
+
+## Rails 3.0.0 (August 29, 2010) ##
+
+*   Added ActiveModel::MassAssignmentSecurity *Eric Chapweske, Josh Kalderimis*
+
+*   JSON supports a custom root option: to_json(:root => 'custom')  #4515 *Jatinder Singh*
+
+*   #new_record? and #destroyed? were removed from ActiveModel::Lint. Use
+    persisted? instead. A model is persisted if it's not a new_record? and it was
+    not destroyed? *MG*
+
+*   Added validations reflection in ActiveModel::Validations *JV*
+
+        Model.validators
+        Model.validators_on(:field)
+
+*   #to_key was added to ActiveModel::Lint so we can generate DOM IDs for
+    AMo objects with composite keys *MG*
+
+*   ActiveModel::Observer#add_observer!
+
+    It has a custom hook to define after_find that should really be in a
+    ActiveRecord::Observer subclass:
+
+        def add_observer!(klass)
+          klass.add_observer(self)
+          klass.class_eval 'def after_find() end' unless klass.respond_to?(:after_find)
+        end
+
+*   Change the ActiveModel::Base.include_root_in_json default to true for Rails 3 *DHH*
+
+*   Add validates_format_of :without => /regexp/ option. #430 *Elliot Winkler, Peer Allan*
+
+    Example :
+
+          validates_format_of :subdomain, :without => /www|admin|mail/
+
+*   Introduce validates_with to encapsulate attribute validations in a class.  #2630 *Jeff Dean*
+
+*   Extracted from Active Record and Active Resource.