patch to add :count=>:words to validates_length_of

This is a patch to add a feature to count words instead of characters, in the ActiveRecord validates_length_of method.

for instance:

validates_length_of :essay, :minimum=200, :count=>:words, :too_short=>"Your essay must be at least %d words."

The patch includes the following tests.

def test_validates_length_of_with_word_count_is
  Topic.validates_length_of( :title, :is => 5, :count => :words )
  
  assert !Topic.create("title" => "this title has too many words").valid?
  assert !Topic.create("title" => "too little").valid?
  assert Topic.create("title" => "this title has just enough").valid?
end

def test_validates_length_of_with_word_count_within
  Topic.validates_length_of( :title, :within => 3..5, :count => :words )

  t = Topic.new( :title => "not enough")
  assert !t.valid?
  assert_equal "is too short (minimum is 3 words)", t.errors.on(:title)

  t = Topic.new( :title => "this title has too many words")
  assert !t.valid?
  assert_equal "is too long (maximum is 5 words)", t.errors.on(:title)
end