fix(verbosity): add some ouput for useminPrepare in non verbose mode

Follow the request from @XhmikosR in #476 in comment:
https://github.com/yeoman/grunt-usemin/issues/476\#issuecomment-61568464

With this last PR I think it should be ok. Only one line displaying information for the user to verify that nothing failed, and if `verbosity` flag set the huge log output as we already know.

/CC @XhmikosR, @sindresorhus