Introduce Pathname.mktmpdir

When I want to create a tmpdir I often want to manipulate it as a pathname. By introducing Pathname.mktmpdir I can get this behavior. 

Currently I must:

```ruby
Dir.mktmpdir do |dir|
  dir = Pathname(dir)
  # ... code
end
```

I would like to be able to instead:

```ruby
Pathname.mktmpdir do |dir|
  # ... code
end
```

ext/pathname/lib/pathname.rb

@@ -603,3 +603,20 @@ class Pathname    # * FileUtils *
   end
 end
 
+class Pathname    # * tmpdir *
+  # Creates a tmp directory and wraps the returned path in a Pathname object.
+  #
+  # See Dir.mktmpdir
+  def self.mktmpdir
+    require 'tmpdir' unless defined?(Dir.mktmpdir)
+    if block_given?
+      Dir.mktmpdir do |dir|
+        dir = self.new(dir)
+        yield dir
+      end
+    else
+      self.new(Dir.mktmpdir)
+    end
+  end
+end
+

test/pathname/test_pathname.rb

@@ -1333,6 +1333,14 @@ class TestPathname < Test::Unit::TestCase
     }
   end
 
+  def test_mktmpdir
+    Pathname.mktmpdir do |dir|
+      assert_equal Pathname(dir), dir
+      assert dir.directory?
+      assert dir.exist?
+    end
+  end
+
   def test_s_getwd
     wd = Pathname.getwd
     assert_kind_of(Pathname, wd)