Fixed mixed tabs/spaces
Remove unused file
Added special comments

In order to test keepSpecialComments option
Update clean-css to v2.0.7

`require('clean-css').process` is removed at the release of v2.0.0.
Instead, now `require('clean-css')` is a constructor of CleanCSS object
and it can be used as `new CleanCSS([options]).minify`.
Merge pull request #3 from shinnn/master

Update clean-css to v2.0.7