Correct File.clone() treatment of File.stats

Cloning the stats object would remove its prototype,
meaning it would lose methods normally available on fs.Stats
instances, such as Stats.isDirectory() or Stats.isFile()

This resulted in errors being thrown whenever a cloned file
was piped through to a gulp.dest() stream when it expected
the isDirectory() method.
Use clone-stats module to clone fs.Stats instances.
Add test for new File.clone() functionality
Merge pull request #8 from hughsk/fix-clone-stats

Correct File.clone() treatment of File.stats