misc: added script to check license header

I will add a job with this script in our continuous integration server.
We should keep our license headers up to date.
app: update license header in util.go
docs: update license headers in outdated files
fs/testing: update license headers
.travis.yml: checking the license header of files
misc/check-license: add license header
misc/check-license: don't check itself
misc/check-license: exit with non-zero status when fail