Track file additions in newly created folders

Others have noted that new files are still not being tracked in newly created folders: https://github.com/gruntjs/grunt-contrib-watch/issues/166 . This fix is an attempt to correct that issue.

I noticed that mkdirThenAddFile behaves differently when fs.mkdirSync is used to create the dummy folder instead of grunt.file.write([folder]/tmp.js). I modified mkdirThenAddFile to use fs.mkdirSync instead. Additionally, I manually vetted the changes by using gaze to watch a folder `gaze.(foldername/**/*.js, ... `, then creating a new folder, then a new .js file in that folder, and finally checking to see that gaze tracked the creation of that new .js file.