Removed requirement for validatable gem.

Allows for using more uptodate versions of the library, or doesn't install it when you're using ActiveModel.

Rakefile

@@ -90,7 +90,6 @@ begin
     s.authors = ["Alexander Lang"]
     s.files = FileList["[A-Z]*.*", "{lib,spec,rails}/**/*", "init.rb"]
     s.add_dependency 'json'
-    s.add_dependency 'validatable'
     s.add_dependency 'couchrest', '>=0.24'
   end
 rescue LoadError

lib/couch_potato/validation/with_validatable.rb

@@ -2,7 +2,12 @@ module CouchPotato
   module Validation
     module WithValidatable
       def self.included(base)
-        require 'validatable'
+        begin
+          require 'validatable'
+        rescue LoadError
+          puts "Please install the gem validatable using 'gem install validatable'"
+          raise
+        end
         base.send :include, ::Validatable
         base.class_eval do
           # Override the validate method to first run before_validation callback