Use verbose for each file wrapping log

Logs is overwhelming when you have to wrap hundreds of files. 
Final log which indicates how many files were wrapped is sufficient for most cases :)

Using grunt `--verbose` flag when you have a problem or want to check what happen.

By the way I've removed the `grunt.log.ok()` call right after `grunt.file.write()` as this function return two (verbose) log messages + `grunt.verbose.ok()`, one for the read operation and another for the write one.