Add sticky? method for File::Stat

This method must return true for Dir::Tmpdir method located in tmpdir.rb
file in the ruby source.
This method from original File.stat has been added in ruby >= 2.0.0.
More, I delete some useless 'puts'.

Github issue: #249
Merge remote-tracking branch 'PierreRambaud/master'

* PierreRambaud/master:
  Add sticky? method for File::Stat

Conflicts:
	test/file/stat_test.rb
	test/kernel_test.rb
Add File.sticky? and make file stickiness always be false.

I did an unscientific check of a bunch of files on my (OS X 10.8) system and found no sticky files.