add tag function. close #1.
add usage()
use getopts instead of getopt due to 1) simplicity 2) bash built-in.
update README.md
complete argv part.
support sending duration, close #2; support log source, close #3; support sending frequency, close #5
support priority, close #6; fix #8;
update README.md
Merge pull request #9 from ypsun/develop

test merge