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
Merge pull request #479 from stephanebachelier/log_useminprepare

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