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
```

1 files changed, 8 insertions, 0 deletions

diff --git a/test/pathname/test_pathname.rb b/test/pathname/test_pathname.rb
index a23dc21ae3..edd96e2724 100644
--- a/test/pathname/test_pathname.rb
+++ b/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)