Started added a validation mixin

Usage:
  class Invoice < CouchRest::ExtendedDocument
    include CouchRest::Validation

    property :client_name
    property :employee_name
    property :location

    # Validation
    validates_present :client_name, :employee_name
    validates_present :location, :message => "Hey stupid!, you forgot the location"

  end

lib/couchrest/validation/contextual_validators.rb

@@ -0,0 +1,55 @@
+module CouchRest
+  module Validation
+
+    ##
+    #
+    # @author Guy van den Berg
+    # @since  0.9
+    class ContextualValidators
+
+      def dump
+        contexts.each_pair do |key, context|
+          puts "Key=#{key} Context: #{context}"
+        end
+      end
+
+      # Get a hash of named context validators for the resource
+      #
+      # @return <Hash> a hash of validators <GenericValidator>
+      def contexts
+        @contexts ||= {}
+      end
+
+      # Return an array of validators for a named context
+      #
+      # @return <Array> An array of validators
+      def context(name)
+        contexts[name] ||= []
+      end
+
+      # Clear all named context validators off of the resource
+      #
+      def clear!
+        contexts.clear
+      end
+
+      # Execute all validators in the named context against the target
+      #
+      # @param <Symbol> named_context the context we are validating against
+      # @param <Object> target        the resource that we are validating
+      # @return <Boolean> true if all are valid, otherwise false
+      def execute(named_context, target)
+        raise(ArgumentError, 'invalid context specified') if !named_context || (contexts.length > 0 && !contexts[named_context])
+        target.errors.clear!
+        result = true
+        context(named_context).each do |validator|
+          next unless validator.execute?(target)
+          result = false unless validator.call(target)
+        end
+
+        result
+      end
+
+    end # module ContextualValidators
+  end # module Validation
+end # module CouchRest