Maven License Plugin was added to add lisense headers to all files

Run `license:verify` to check whether there are any files without header
or `license:format` to add the header to all the files.

Notice list of the exclusions.

Changed headers will be committed separately so that we don't mesh up
our commits.
Added license headers for all source files

Done by `mvn license:format`. Markdown files are to be fixed separately if
any discrepancy found because they are not managed by license plugin.