[thoughtbot#131] Refactor validate_presence_of_matcher

I'll be adding a number of new options, so I'm refactoring to use options hash
so that all of the arguments can be stored in once place.

lib/shoulda/matchers/active_model/validate_presence_of_matcher.rb

@@ -21,15 +21,20 @@ def validate_presence_of(attr)
 
       class ValidatePresenceOfMatcher < ValidationMatcher # :nodoc:
 
+        def initialize(attribute)
+          super
+          @options = {}
+        end
+
         def with_message(message)
-          @expected_message = message if message
+          @options[:expected_message] = message if message
           self
         end
 
         def matches?(subject)
           super(subject)
-          @expected_message ||= :blank
-          disallows_value_of(blank_value, @expected_message)
+          @options[:expected_message] ||= :blank
+          disallows_value_of(blank_value, @options[:expected_message])
         end
 
         def description